The 35-year-old scored 28 goals in 46 appearances for Man United 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 has stayed at the club to continue his rehabilitation.
Earlier this week, it was reported that Mourinho wanted to keep Ibrahimovic after being impressed with the striker's recovery from injury, and the Portuguese has now had his say on the situation.
"Possible, it's open, of course we wanted to be honest and open and left everything for him to make the decision," Mourinho told reporters.
"At the same time let's make a decision that makes us all happy and if that is for him to stay and for us to wait, let's say in December maybe, because I don't think before December he can get back to competitive football but why not wait for a player who gave us so much, we are changing ideas and it's with Mr [Ed] Woodward and Zlatan's agent but the final decision is the best for the player."
Man United arrived in the United States last weekend as they prepare to begin their pre-season tour of North America, which will include matches against the likes of Manchester City, Real Madrid and Barcelona.