David Beckham said Manchester United have lost "a great leader" following Roy Keane's decision to leave the club.
"The fans will be upset because they loved him down there - and so did I when I played there," he said.
"He was a great leader. The best thing about Roy Keane is that you knew where you stood with him.
"If you did something wrong he'd tell you but if you did it right then he'd also tell you - in his own way. Every player looks up to him for leadership."
Beckham, who played alongside Keane for several seasons in United's midfield, said the Irishman's input was one of the reasons for the club's success in recent times.
"It is a big shock because I thought he'd stay there for a few more years, and go into coaching maybe, but he's made his decision and everyone will respect his decision," he said.
But while Beckham was shocked, Steve McClaren, former assistant boss at United and now manager of Middlesbrough, said only the timing of the announcement came as a surprise.
"Every relationship ends and you move on quickly. It was never going to go on forever," he said.
"I feel privileged I had three years working with him. He certainly opened my eyes to what it takes to be a top athlete at a top club and I've taken many lessons from working with Roy."
Arsenal boss Arsene Wenger believes Keane's departure is a huge blow to United.
"He was certainly their most influential player in the last 10 years," said the Frenchman.
"It's a complete surprise that he's gone in the middle of the season. It looked like he was a bit upset recently. Why, I don't know."
Asked whether Keane might go to another Premiership club, Wenger replied: "I wouldn't be surprised."
However, Wenger laughed off suggestions that Keane might be Highbury-bound.
"Not at the moment, no," he added. "We have put our confidence in our players and will keep doing that."
Struggling Portsmouth became the first club to show an interest in signing the 34-year-old midfielder.
A statement from the club read: "Following instruction from Portsmouth chairman Milan Mandaric, the club's chief executive Peter Storrie is making enquiries about Keane's availability.
"Mandaric believes Keane is exactly the kind of positive character Portsmouth would welcome."
The announcement of Keane's departure came just over three weeks after the skipper gave an interview to MUTV in which he criticised several team-mates.
The interview was never broadcast but Michael Crick, Sir Alex Ferguson's unofficial biographer, thinks Keane went too far for the United manager.
"Ferguson laid down very strict rules for all his players but then he made exceptions for the really great players like Cantona and Keane," Crick told BBC Radio Five Live.
"Ferguson gave Keane a huge amount of leeway. He turned a blind eye to some of the things Keane's been up to and some of the comments he's made.
"We don't know the full story but it may be that Ferguson just decided that this was a move too far by Keane and he had to go."
Tony O'Donoghue, sports editor of Irish national broadcaster RTE, went further.
"This was a game of poker that went too far for United," he told Five Live.
"Roy was not going to have a contract extension or a contract renewal and basically I think we've seen Roy Keane sacked."
Paul Parker, a former team-mate of Keane's at Old Trafford, is not convinced the MUTV interview was solely to blame.
"There's more to it than that," he told News 24.
"That didn't help him but that was Roy and I think the club and the boss understood that.
"But I think a lot of it all was down to what he wanted to do and it wasn't that one isolated situation."
Keane had already stated that he did not expect to sign a new contract with United but Ray Wilkins said he was still "flabbergasted" by Friday's announcement.
"This will be a total shock to everyone in football," the former United midfielder told BBC Sport.
"There was a lot of talk about him leaving the club in the summer, but for him to go before the January transfer window is a major surprise.
"Roy was an extremely powerful figure at Manchester United and an exceptional player. I'm flabbergasted."
Wilkins added that he would not be surprised to see Keane now join Celtic, the club at which the midfielder once stated he wanted to end his career.
"Roy has publicly stated that he wouldn't want to play against United in the Premiership, so that might be the obvious choice for him," he said.
"It would be interesting to see Roy working with Gordon Strachan. Two Celts together might be a bit fiery, but he survived 12-and-a-half years with Sir Alex so I'm sure he wouldn't have a problem."
Lou Macari believes Keane will first need time to come to terms with the end of his United career.
"I was there for 11 years, but the day they tell you your career is no longer going to be with Manchester United, it's a real bolt from the blue," the United legend told BBC Five Live.
"It takes a while for you to recover from it and I would think Roy will be no different at the moment."
Keane's last game for United was against Liverpool in September, when he broke his foot.
Since then, United have endured a torrid time, losing to Middlesbrough in the Premiership and Lille in the Champions League.
Andy Mitten, founding editor of the United We Stand fanzine, said Keane would be sorely missed.
"I feel he had something to contribute. He was such a big influence on the club and the fact he is driven by success would only have been a good thing for the club," Mitten told BBC News 24.
"He's the best player I've ever seen in a Manchester United shirt, the driving force behind much of the club's success since he joined in 1993 and a legend in the eyes of most United fans."
Mark Longden, chairman of the Independent Manchester United Supporters' Association, is angry the club did not do more to keep the midfielder at the club.
"I'm absolutely staggered," he said.
"We've been trying for years to replace Roy Keane and then we get rid of the one we've got in the middle of the season. It seems absolutely crazy to me."
"The season seems to be going from moderate to worse."
Reaction from fans on the BBC's 606 messageboards was also mixed, but many are upset they will not get a chance to give Keane a send-off.
"He may not be the player he once was but he was still our best midfielder and the best captain in the league," read one post from Black_Bag-TFBBL.
"If he was leaving at the end of the season at least we could say goodbye to him properly from the stands. This is the wrong way to do it."