The 35-year-old scored 28 goals in 46 appearances for the Red Devils during the 2016-17 campaign, but he missed the final stages of the season after picking up a serious knee injury.
The 20-time English champions initially opted against renewing the striker's contract for a further 12 months, although the Swede stayed at the club to continue his rehabilitation.
Speculation that Ibrahimovic could re-join Man United gathered pace in recent days, and the club announced on Thursday that the forward has signed a fresh 12-month contract after overcoming the serious knee problem.
It had been claimed that Ibrahimovic could return to action as soon as October, but Mourinho has hinted that the Swede will not feature until early 2018.
"I don't want to think about it, I don't even ask, speak about it or with the medical team about it," Mourinho told reporters.
"He will be an extra man for us in the second part of the season, when I say second part I say normally we say January after Christmas, when the transfer window reopens in January so that's normally but I have no idea at all and I think the right way is not even to think or speak about it and just let him do his work step by step and comeback when he's ready.
"He will not be ready for the group phase of the Champions League, I don't think there is any chance of that do we have space in the Champions League list to have him? Yes, so we don't need to leave any player outside of the list. But I'm not thinking Zlatan to play any part of the group phase hopefully he can play in the knockout phase but for that we need to finish top two."
Ibrahimovic will wear the number 10 shirt for Man United, who have opened their new Premier League season with back-to-back wins over West Ham United and Swansea City.