The Swedish striker was United’s top scorer last season with 28 goals in all competitions.
But he suffered a serious knee injury during the Red Devils’ Europe League quarter-final match against Anderlecht at the end of April.
United decided not to renew his contract and he became a free agent during the summer.
It was feared the 35-year-old may never play again but over the summer he has posted a series of training videos showing him fighting fit.
Yesterday, Ibra posted a clip showing him smashing a roundhouse kick into a boxing bag in the gym, before reversing the movement and catching the bag with his left foot as well.
The video has been seen over three million times with eager fans praising his dedication to recovering.
One social media user wrote: “Welcome back Zlatan, stay please, I miss you, beast.”
Others simply said: “Welcome back to Manchester.”
Jose Mourinho revealed talks over a new deal are already underway.
And reports have suggested United could even finalise a new contract for Ibrahimovic as soon as this week.