"Four mistakes for the goals. We have to be self-critical for that. We controlled the first half hour but made some poor decisions. We should be in the lead at the break," said Frank Lampard.
"They have pace and if you turn over the ball and allow them to run in behind you they can hurt you.
Champions League
Real Madrid will win the trophy, there's nothing you can do about it - The Warm-Up

Tammy Abraham lors de la rencontre Manchester United-Chelsea / Premier League

Image credit: Getty Images

We were comfortable in the first half. But there are lessons there. You can't make mistakes at this level. But if you look at the game, it is not a 4-0 game.
It was an awful start for the new Chelsea boss in his first competitive game on the bench for his old club who struck the woodwork twice in the first half but paid for their defensive frailties as Anthony Martial and Dan James also scored.
Eurosport takes a look at seven key things we learned from a one-sided contest...

Rashford ready to take on more responsibilities

Marcus Rashford of Manchester United celebrates after scoring his team's third goal during the Premier League match between Manchester United and Chelsea FC at Old Trafford on August 11, 2019 in Manchester, United Kingdom

Image credit: Getty Images

There were question marks when United allowed Romelu Lukaku to leave Old Trafford without a replacement on transfer deadline day, but this was an emphatic response from the two forwards left to fill his shoes.
Rashford was the man on the spot when United completed that famous back-from-the-death victory over Paris Saint-Germain last season in the Champions League, a result that virtually sealed the full-time gig for Ole Gunnar Solskjaer.
And after a season in which Paul Pogba drew further attention to himself with his bizarre quickstep run-up from 12 yards, this was another no-nonsense approach with Rashford back on penalty duties.
Rashford has now scored all four of the penalties he has taken in competitive senior matches for club and country (including shootouts).
Putting his foot through the ball in much the same way as he had done at the Parc des Princes, the opening goal eased the pressure that was palpable in the opening quarter of an hour, and the manner of the strike told us that the 21-year-old is ready to become United's main man.

Marcus Rashford of Manchester United celebrates scoring their first goal during the Premier League match between Manchester United and Chelsea FC at Old Trafford on August 11, 2019 in Manchester, United Kingdom

Image credit: Getty Images

Azpilicueta exposed in Luiz absence

Despite the youthful line-ups leading to several unforced errors, it was the failings of perhaps the most experienced player on the pitch that resulted in the game drifting away from Chelsea.
Where Tammy Abraham got very little change out of Harry Maguire, Cesar Azpilicueta was pinned and bullied off the ball by Martial for United's second as he got in front of the Spaniard to meet Andreas Pereira's low cross.

Anthony Martial lors de Manchester United-Chelsea / Premier League

Image credit: Getty Images

Perhaps still playing on his mind 17 seconds after the re-start, Azpilicueta played Rashford onside from Pogba's raking pass, and the defender was emphatically punished.
David Luiz's departure and Antonio Rudiger's lay-off has placed extra pressure on the 29-year-old to help the young pairing of Kurt Zouma and Andreas Christensen - and he didn't pass his first test of the season as Chelsea's most senior outfield player.
With the Blues embarking on an uncertain period that has forced a young set of players onto their new manager, it was a far from ideal start for the Chelsea captain who will now be tasked with shackling the formidable Liverpool attack in the UEFA Super Cup on Wednesday.

It was one of Azpilicueta's worst games in a Chelsea shirt

Image credit: Getty Images

A game too soon for Mount?

Despite the alarming and somewhat flattering scoreline, there was very little wrong with Chelsea's first 45 minutes on Sunday. Abraham and Emerson Palmieri both struck the woodwork after two well-worked moves, and Chelsea's play was pretty in patches.
While Ross Barkley continued his encouraging form from pre-season, forcing David de Gea into one of those sprawling trademark saves with his feet, the game looked like it had come a little too soon for Mason Mount.

Mason Mount

Image credit: Getty Images

Aaron Wan-Bissaka earned his £50m move off the back of being consistently excellent in not allowing opponents past him down the right side, and Mount frequently positioned himself inside, unwilling to take on the athletic right-back.
Christian Pulisic replaced Ross Barkley in a straight swap down the left, and the American immediately showcased his brilliant ability to turn and go as he rolled Wan-Bissaka and was body-checked with his opening touches.
Despite Chelsea dominating possession, with Jorginho and Mateo Kovacic at the base of the midfield, Mount's inability to have an impact when given creative licence was further highlighted by United's clinical counter-attacking.
Handed a baptism of fire by a man who clearly has plenty of faith in his skills following their link-up at Derby, Mount conceded possession on 16 occasions - only Azpilicueta (22) did so more frequently - and the game in essence passed him by.
N'Golo Kante's return to fitness may mean it is a while before we see the talented youngster starting again in the Premier League.

McTominay's example should encourage Lampard

A positive takeaway from the opening weekend of the Premier League season was the number of English starters at the top six clubs - the most in nearly six years.
Chelsea fielded their youngest starting XI (average age: 25 years and 199 days) since November 2012.
Meanwhile, the average age of United's starting XI was 24 years and 227 days; the youngest of any team in the opening round of the season.

Frank Lampard and Ole Gunnar Solskjaer offer instructions

Image credit: Getty Images

Taking into account the starting line-ups fielded by Manchester City, Arsenal, Liverpool and Tottenham, with an average age of 23.5 years, there hasn't been this many English starters among the Big Six, with an average age so low, since early 2007.
Pereira and Scott McTominay both started for United that memorable night in Paris in the Spring, while the latter also started at the Nou Camp in what proved a chastening evening.
Lampard is in a position where he will have to keep blooding youth during a difficult transitional period in much the same way that Solskjaer did, and the growing maturity of McTominay in particular should serve as an example to the likes of Abraham and Mount moving forward.

Maguire makes imperious start

In a contest when both teams gave away the ball many times in poor areas, Chelsea managed to find space out wide - but seldom did Abraham get change out of £80m man Maguire, who showed confidence to handle the big occasion, and the pressure of being the world's most expensive defender.
After handling one or two situations where the United defence had been put under pressure by those in front of them during the first period, Maguire continued his strong debut after the break by stepping in front of Abraham to start off another attack, leading to Jorginho being forced into hauling down Jesse Lingard, but not before Maguire had given Azpilicueta a whack when releasing his pass.

Harry Maguire

Image credit: Getty Images

United kept just two clean sheets in the Premier League at home last season, but De Gea now has far more faith in who's in front of him, and it was Maguire who began the attack for United's crucial second goal, stealing the ball again off Abraham.
The 26-year-old had his name sung by home fans throughout, and he made more clearances (seven), more interceptions (four) and more blocks (two) than any player on show.
Rafael van der Vaart revealed on Saturday that the former Leicester man was considered England’s worst player at the Nations League - but on this evidence, he looks worth every penny.

Maguire rises to win the ball from Christian Pulisic

Image credit: Getty Images

Martial relishing being back in the limelight

Martial completed four take-ons in the first half against Chelsea, twice as many as any other player on the pitch, and he joined Rashford in having the most number of shots of any player during the 90 minutes (four).
Back wearing the No 9 shirt, and back running at defenders, his 10 sprints were only eclipsed by Luke Shaw (12) and Pereira (11), and after a pre-season in which much was made of the fitness work conducted by Solskjaer, it paid dividends as United roared through the gears in the final third.
Having soaked up a period of Chelsea pressure in the opening 20 minutes of the second half, the logic behind James' signing in tandem with the lightning pace of Rashford and Martial make the United forward line a devastating prospect when things click.
With the often lumbering presence of Romelu Lukaku offloaded, this was a reminder that at 23, Martial still has plenty of time to become the poster boy of United's return to former glories.

Anthony Martial (Manchester United)

Image credit: Getty Images

Pogba shows his world-class worth

A divisive character to put it kindly, the game was screaming out for a sprinkling of stardust after an hour.
At that stage, the over-arching feeling from this contest between two Premier League behemoth's of yonder years was that neither side were Champions League class.
But after Chelsea had come no closer to converting their superior possession into parity on the scoreboard, it was Pogba who once more underlined why he is worth all the fuss.
No player conceded possession more times than the former Juventus midfielder (28), but that demonstrated his constant desire to find the killer pass. Eventually he found it, and in some style.
Moments after Martial had stroked in the second, the France World Cup winner produced the ball of the match to send Rashford on his way for his second.

Paul Pogba et Mike Phelan

Image credit: Getty Images

Not satisfied, Pogba strode forward onto a Martial lay-off to unselfishly pick out the perfect weighted pass for his second assist as James capped his Premier League debut with United's fourth.
He may no longer be on penalty duties, but the man who was the subject of much interest from Real Madrid this summer showed he still has a major part to play in Solskjaer's rebuild.
Pogba joined all his team-mates in greeting each of United goals with a huddle, and the rallying cry of McTominay that was picked up on the television cameras showed that while several of the 'dead wood' remains at the club, the squad remains united.
- Ben Grounds
Opinion: Cavani has given United an elite striker's blueprint, now they need to sign one
Premier League
Man Utd ditch red banners at Old Trafford to improve home form - Solskjaer