Ibrahimovic, 36, is out of contract at Old Trafford at the end of the current campaign and has been heavily linked with a move to MLS club Los Angeles Galaxy.
The former Paris Saint-Germain forward has not played for United since their 2-2 draw against Burnley on December 26, having decided that he was not in the right shape after recovering from career-threatening cruciate knee ligament damage sustained in April.
Ibrahimovic has made only seven appearances for United this term, scoring just once in a shock League Cup loss at Bristol City.
He hopes to return in time for the second leg of the Champions League last-16 tie against Sevilla at Old Trafford a week on Tuesday, but United boss Mourinho said the Swede’s long-term plans may lie away from Manchester.
“For Zlatan, we all think it’s his last season at Manchester United and it will be a very personal decision for him to play or to stop,” Mourinho told reporters on Friday.
“I think he won the right to choose his life, his future, such an amazing player and an amazing career.
“Only that awful injury in the wrong moment broke a fantastic couple of seasons he could and should have with us.
“This season has been really difficult for him. He’s not injured. Does he feel totally happy, ready and convinced that he’s in the condition to help the team in this moment? No.
“But he’s such an honest guy and such a champion he only wants to be back with that feeling of: ‘I’m totally ready for it’.
“So he’s working hard and hopefully his evolution brings him to that level he wants to have a positive answer.”
– Fellaini future in the balance –
Ibrahimovic might not be the only high-profile departure from United this summer.
Mourinho, whose side start the weekend in second place as they prepare for Monday’s Premier League trip to Crystal Palace, has admitted that Marouane Fellaini’s future also remains in the balance.
The Belgium midfielder is out of contract at the end of the season, and is reported to have rejected two offers to extend it.
Fellaini, 30, has not played since suffering a knee injury that needed surgery against Tottenham at the end of January, but is expected to be fit for the Champions League quarter-finals in early April if United get there.
“He’s still a young player. His injury had minor surgery and he will be back to play again in a couple of week,” Mourinho said.
“So if we proceed in the Champions League after Sevilla, I think he will be ready for a hypothetical quarter-final, a hypothetical FA Cup semi-final, and for the last couple of months of our Premier League fight.
“Is he going to stay or leave? I don’t know. I want him to stay, the club want him to stay.
“But with the process, the negotiations, the numbers, and the distances between the offer and what he wants, I really don’t know.”
Mourinho added that he was uncertain as to whether England boss Gareth Southgate has met Scott McTominay over his international future.
Lancaster-born McTominay, 21, is also eligible for Scotland through his father, and it is understood that Southgate is keen to get the midfielder to commit to England.
“I don’t know if that is true. I can’t confirm because I have no information about that and my advice to him is not about national teams,” Mourinho said.
“My advice is to keep being the way he is: Very humble, very open to learn, intelligent. He has to keep going in the same direction, with the same profile, trying to fight to have minutes with Manchester United, which is not easy.”