The Swede watched his former team-mates train at the UCLA college campus, near where his current team Los Angeles Galaxy are based.
Addressing the club's Instagram followers, Ibrahimovic said: "Hi everybody, guess who's back. Watching United train I see a lot of quality but not quality like me!
"It's great to see the guys and to see them happy and training good. Be ready for this season because something special will happen and the boss has something going on. So all the best for this season and enjoy!"