Arsenal boss Arsene Wenger was taken aback at the news of Roy Keane's exit from Old Trafford but does not expect the announcement to mark the end of the Corkman's playing career.
"It's a big surprise to let him go in the middle of the season," the Frenchman told BBC Radio Five Live.
"It looked like he was a bit upset recently. Why, I don't know. You must know what is going on inside the club to judge that."
Wenger observed that Keane no longer has the physical attributes which made him a fearsome opponent in his late twenties, but added: "He's still an influential (team) member when he plays and has a huge football brain."
As for whether Keane may return to the game in the Premiership, with one of United's rivals, Wenger said: "I wouldn't be surprised."
But Keane's destination will not be Highbury, where Wenger has invested considerable trust in a young midfield this season.
"At the moment, no. We have put our confidence in our players and will keep doing that," he said.
Former United and Republic of Ireland striker Frank Stapleton believes Keane's recent criticism of team-mates in an MUTV programme, which was considered so strong that the show was pulled from the schedules, was 'at the centre' of the mutual decision to quit.
"In some ways I'm surprised and in other ways I'm not," Stapleton told Sky Sports News.
He continued: "It is a reaction to what happened a few weeks ago, when he criticised his team-mates publicly, plus a combination of the fact that maybe they weren't ready to renew his contract at the end of the season. He probably said 'I'll go now rather than wait'.
"The programme is the centre of this. Just before (the pulled MUTV programme) he had said he didn't think he'd be at the club after this season maybe to force them into giving him a contract.
"At this stage of his career Roy knows it is very straightforward. He wouldn't have thought twice about saying 'right, let's call it a day now, let's move on' and the club have obviously made the decision not to renew his contract."
The Dubliner concluded: "In the period that he has been there he has had unbelievable success. But in my mind no player has ever been bigger than Manchester United. The club will go on and look to get stronger."
Another former United player, Lou Macari, was shocked by the news. He admitted: "It's a real bolt out of the blue. I've always thought of Roy Keane as a major part of Manchester United.
"There is always a place for Roy Keane in the team, and it is only when the likes of Keane, Paul Scholes and the Nevilles have all gone that people will realise their value.
"Trying to replace Roy Keane is almost impossible and no matter what Roy will say over the next few weeks and months ahead, Roy will be gutted that he is no longer a Manchester United player."
Nicky Butt, Keane's former midfield partner who is now at Birmingham City, believes the Irishman will be remembered as a leader.
He told Sky Sports News: "He was one of the best players the club has ever had. And he was a leader - that's how he will be remembered."
However, he denied he was irreplaceable, adding: "Ten years ago you could have said that about Bryan Robson."
