Rio Ferdinand will replace suspended skipper Nemanja Vidic against Otelul Galati on Wednesday with Sir Alex stressing the 32-year-old defender is in the process of altering the style of his game.
The classy centre-back, a £30million signing from Leeds, is facing competition for his place from the likes of Jonny Evans, Chris Smalling and Phil Jones but has proved on numerous occasions that his experience is vital.
"I explained to Rio the other day simply this - we have two young centre-backs coming through and I'm happy to have four and hope I can keep four of them for a long time," said the boss at his press conference at Old Trafford.
"Rio's almost 33 and has obviously lost the yard of pace he had five years ago but that doesn't mean to say he can't tailor his game into a different game where he doesn't rely on pace. We've all faced that, the decisive moment in your career when you realise you have to change your game. I did that myself when I lost some sharpness so I tailored my game differently. Rio will do the same and he'll be fine."
The boss is full of admiration for Ferdinand and feels the former West Ham defender has plenty of gas left in the tank when it comes to top-level football.
"I don't think there will be any problem," he added. "In his time with us, eight years or so now, he's been a fantastic, consistent performer, top-class and absolutely brilliant. Rio will rearrange his game a little bit but not drastically. He's still one of the best footballers in the country in terms of his use of the ball. He has a great presence and can still tackle and head the ball.
"All that's changed, and maybe he's realising it now, is the electric pace he had two or three years ago. He'll change, there's no problem with that, and he'll play a big part."