The Swedish striker rejoined Mourinho's side in August having been released at the end of last season following a cruciate knee ligament injury he suffered in April.
Mourinho had previously suggested that Ibrahimovic, who is still working his way back to fitness, had no chance of playing a part in the club's Champions League group stage campaign and would not be fit until the second half of the season
He said: "I am not thinking Zlatan will play any part in the group phase.
"I don't think there is any chance of that. Do we have space in the Champions League list to have him? Yes, we have.
"Hopefully he can play in the knockout phase but for that we need to finish top two."
United will start their Champions League campaign at home to FC Basel on September 12 before away trips to CSKA Moscow on September 27 and Benfica on October 18.
They then host Benfica at Old Trafford on October 31 before travelling to Basel on November 22, finishing with a home match against CSKA on December 5.