The midfield maestro has been a major influence in the charge towards a 20th title since coming out of retirement in January.
However, the veteran missed the midweek defeat at Wigan with fellow veteran Ryan Giggs taking his place alongside Michael Carrick in the engine room.
Speaking at Friday's press conference, Sir Alex stated: "Paul has shown his influence since he has come back. We did the right thing resting him on Wednesday. It may not have looked like that to the fans but, after playing on Sunday and then having another match three days later, we did the right thing. He will play on Sunday and he will be fresh."
The boss was not concerned with any fresh attempt at mind games from Manchester City chief Roberto Mancini, who claimed after his team's win against West Brom that five points is too big a bridge to gap at the Barclays Premier League summit.
"I think he has maybe picked the wrong time to say it," explained Sir Alex. "There are still five games to go. The twists and turns of the Premier League are fascinating. We experienced that last year. We had to play a vital game against Chelsea and won. Experience helped us that day. Hopefully it does again."
"Sunday's game is the one to focus on."