Alex Ferguson was left stunned when then-Manchester United boss Louis van Gaal sold Jonny Evans in 2015, according to Michael O'Neill.
Evans joined West Brom at the age of 27 for a reported fee of £6million, having won three Premier League titles with United, where he came through the youth system.
Northern Ireland boss O'Neill has now revealed that Ferguson made his disbelief at Van Gaal's decision clear, as he felt the defender would have seen out the majority of his career at Old Trafford.
"In September after last year's Euros, there was a conference in France for the coaches and Sir Alex was there talking about it and he openly said to me that he could not believe Van Gaal had sold Jonny from United," O'Neill told the Belfast Telegraph.
"Having seen him come through, Sir Alex felt Jonny would spend his whole career at United or certainly to the latter stages of his career.
"I think there were a lot of factors why he ended up at West Brom. In the previous two years he had an injury and at Manchester United they had Rio Ferdinand and Nemanja Vidic.
"Then Jonny and there was a succession plan that didn't happen because of the change in manager.
"At that time he was on the radar of other big clubs. I know that for a fact but they possibly had reservations about him because of the injuries he had and because he maybe hadn't played as much in the previous 18 months. That was just unfortunate."
Evans, now 29, has been linked with a move to Leicester City. The 2015-16 Premier League winners have reportedly had a £10m offer rejected, with O'Neill talking up his quality.
He continued: "When I look around the Premier League and see British centre-backs, I don't see anyone better.
"I look at Gary Cahill at Chelsea and Chris Smalling at Manchester United and Jonny Evans is every bit as good as them.
"His move away from Manchester United to West Brom was maybe the right thing for him at that stage in his career but he is a player who I believe can play right at the top of the Premier League.
"He was an absolute bargain at £6million which I believe West Brom paid for him and if you are asking me is he worth £20m, I think he is a bargain at that price.
"Given that people are talking about Southampton's Virgil Van Dijk as a £50million player, that makes Jonny Evans sound cheap at £20m. If he goes to Leicester I think it is a good move for him."