The New York City wideman had earned his first call up for the under-21s after impressing in the United States.
Ex-Manchester United youngster Harrison - who paid £1,200 for a new flight himself after initially missing his first from the US at the weekend - could make his debut in the Euro 2019 qualifier against Scotland in Middlesbrough on Friday.
He has scored 10 goals and contributed six assists this season with City second in the MLS Eastern Conference to earn the nickname 'Golden Boy' from his team-mates, who include World Cup winners David Villa and Andrea Pirlo, to leave Boothroyd impressed.
He said: "It (the nickname) must be nice for him. It shows you the pathway has changed or is changing. There's no hard or fast route (for call-ups). Some go abroad and play and now we have someone who goes away, ends up playing for New York City and does really well.
New York City FC midfielder Jack Harrison (11) celebrates scoring a goal with FC forward David Villa (7) and FC defender Rodney Wallace (23) against the New England Revolution during the first half at Yankee Stadium.
Image credit: Getty Images
"He's been doing really well for New York and he's been playing with one or two reasonable players after the tweets we've seen. He's one we've been aware of for about a year. He first came on our radar from some of the scouting, we had a look and did our due diligence on him.
"If his stats are where they are in terms of his goals and assists, and we've watched a few games remotely, he looks like someone who could add to us."
Should Stoke-born Harrison - who moved to the States in 2011 - not feature on Friday, he is likely to play in the Group 4 game in Andorra on Tuesday with Boothroyd confirming he is set to hand a debut the 20-year-old over the two matches at some point.
Boothroyd added: "He looks better than fine in the group, especially as he's just got off a plane. He actually missed his first plane but we managed to get him in. He's spent most of his time asleep - although that may be to avoid my meetings. He's a good sort."
But Boothroyd is without Ben Chilwell (knee), while he is also waiting to discover if Ainsley Maitland-Niles (knock) and Jonjoe Kenny (back) will be available.