Manchester United legend Denis Irwin feels Manchester City's emergence will keep Sir Alex Ferguson from retiring.

Former defender Irwin, who enjoyed a significant share of the glory as a United player between 1990 and 2002, has been particularly impressed by the 69-year-old's ability to keep regenerating dynamic teams over the years, as well his determination to overcome each new major rival that has come along.

Irwin told the Manchester Evening News: "I think it is incredible in this day and age to look after a top club for so long and to go through so many teams within that 25 years.

"There was the powerful side in the early 90's, then all the young lads came through in the mid-90's.

"Even since then he has had to change players, but there has been success all along the way. I think the way Manchester United play football as well, they try to entertain, playing with wingers. I think it is phenomenal what he has done there over a very successful 25 years.

"Certainly there have been challenges along the way and when he came, United hadn't won the league for such a long time. Liverpool were the successful team in the late 80's, so that was his first challenge.

"Then Leeds was the challenge, then Blackburn spent a bit of money and won the league in 1995 and that was the next challenge.

"For the remainder of the 90s it was Arsenal, then Chelsea came aboard and of course, in the last couple of years, the next challenge has become Manchester City.

"That is going to be a big one because of the amount of resources they have - so I think he will be going on for quite a while yet."