The striker who arrived at United in July 2009 played 52 games for the Red Devils, scoring 17 goals in the process.
Sir Alex told ManUtd.com:
“It is a pleasure to have had Michael at Manchester United. Michael is a consummate professional and was an influential figure in our dressing room. Unfortunately he had no luck with his injury this season.
“Michael’s record of goals per game is exceptional and we wish him all the best for the future.”