Former Rangers, Everton and Scotland boss Walter Smith has undergone an operation on Monday and is currently recovering in hospital, the Ibrox club announced on Saturday.
Rangers have said that the 73-year-old's family "have requested privacy at this time".
Smith was Sir Alex Ferguson's assistant manager at Scotland for the 1986 World Cup and took charge of the national team in 2004.
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Jonathan Burns, son of famous Celtic idol Tommy, sent a get well soon message to Smith.
Gerrard asks fans not to travel for Old Firm
Rangers manager Steven Gerrard has urged fans of the Scottish Premiership champions to remain at home amid the Covid-19 crisis and not follow them to Parkhead when they face bitter rivals Celtic next Sunday.
Thousands of Rangers fans gathered at the club's stadium, training ground and George Square in Glasgow's city centre in violation of lockdown rules as they celebrated the team's 55th league crown when Celtic drew with Dundee United.
Scottish police arrested 28 football fans and fined several others last week following wild celebrations sparked by Rangers' first league triumph in 10 years.
The gatherings were met with anger by the general public, with Scottish First Minister Nicola Sturgeon saying fans risked the country's progress in containing the novel coronavirus and described the scenes as "infuriating and disgraceful".
"It is a great privilege to be the manager of this great football club, and with that comes great responsibility," Gerrard wrote in a letter on the club's website.
"I believe we must take collective responsibility to ensure we uphold the good name of this 150-year institution which represents far more than 11 players on the pitch.
"The players would want nothing more than to hear the fans sing and support us in the ground ... Sadly, right here and right now, you can't be there with us. You can't follow us to Parkhead as champions - and the first time we can say we are champions in a decade."
Additional reporting from Reuters.
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