The 26-year-old joined Newcastle for £6million last week.
United boss Fergie said: “Alan’s been a fantastic character in the dressing room and a very important player for us since he came here.
“I was very sad to see him go in many ways.
“There are few characters as big as him in this country at the moment and it was a very difficult one for us to decide.
“He’ll be remembered with great affection by everyone here and I’m sure that’s the case with the fans too.
“I don’t think there has been a more popular player with the supporters during his time here.
“And also he’s been a really good ambassador for this club so we wish him well.”
