Sam Allardyce and Mark Hughes both wished Alex Ferguson well after the former Manchester United manager was taken ill on Saturday.
Ferguson, the most successful manager in Manchester United's history, underwent emergency surgery on Saturday for a brain hemorrhage, the club said.
"The procedure has gone very well but he needs a period of intensive care to optimize his recovery," a statement said. "His family request privacy in this matter."
Everton manager Allardyce, who had just seen his side score a last-minute equaliser against relegation-threatened Southampton, described Ferguson as a "personal friend" and hoped the operation was a "major success".
Alex Ferguson in hospital: Football unites in wishing former Manchester United manager well
Southampton's Hughes once played for Ferguson at Manchester United, helping the club win his first English Premier League title in the 1992/93 season, and echoed Allardyce in wishing the 76-year-old Scotsman well.
Ferguson's son Darren, the manager of Doncaster Rovers, was reported to have missed his team's match on Saturday for family reasons.
Ferguson was United manager from 1986 to 2013, winning the Uefa Champions League twice, the Premier League 13 times and five FA Cups.