Ferguson (left) said it was difficult to part with a player he admires

"Roy will go on to be a manager, there is no question about that," said Ferguson. "He is very driven, a great influence and very determined.

"When I look at that I can see myself in Roy and Roy in me.

"He was fantastic to me, the best player I had, but I now see him going on to the next stage in his career and that is becoming a manager."

Keane left United in November 2005 after 12-and-a-half years at Old Trafford and signed an 18-month deal with Celtic in December.

It is tough when the time comes to say goodbye Sir Alex Ferguson

The 34-year-old had a contract at United that was due to expire at the end of the current season but left after his relationship with the club had deteriorated.

The midfielder criticised the team in an interview on MUTV, which was never shown, and that appeared to have brought forward his departure.

Ferguson added that the decision to part with a player with whom he had such a close association was incredibly difficult.

"When you have guys who have given you such fantastic service it is tough when the time comes to say goodbye," added Ferguson.

"It was time to go, it was time to part and the problem is when you fall in love with one of your players because of what they've done with you.

"Players almost become like family but when you get to that age they you have to make that terrible decision and change things."