McClaren wrote in the Daily Express: "Van Persie has been the missing link United required, a natural goalscorer and a hell of an all-round player as well.
"United needed that ruthless streak in the penalty area and Sir Alex fixed it. That's where he is brilliant - at seeing a problem and getting it fixed.
"Where do United go from here? They will keep winning titles and cups and Sir Alex will carry on for as long as he wants. The way he handles the pressure of running one of the biggest clubs in the world is a lesson to us all."