The striker ruptured his cruciate ligament in the final moments of normal time as United defeated RSC Anderlecht 2-1 in extra-time to book a place in the UEFA Europa League semi-finals.
The Swedish striker jumped to challenge Kara Mbodj for the ball in the air as United searched for a late winner but landed very awkwardly on his right leg, and was evidently in agonising pain as he recieved treatment on the floor before being escorted off by medical staff.
Mourinho "not optimistic" about the injury
If it is the cruciate ligament that Ibrahimović has damaged, he could be looking at seven to nine months on the sidelines and with a contract extension still not agreed with the club it's possible that fans have seen the last of the Swedish striker at Old Trafford.
Manager and close friend of the 35-year-old José Mourinho was not getting his hopes up after the full-time whistle as he told reporters that he was "not optimistic" about the extent of the injury which led to more speculation that a new player will be leading the line for the Reds next season.
Mourinho was obviously in an unpleasant mood after his side's somewhat poor showing on the night but definitely would have had the fact that he's likely to be missing his top scorer for the rest of the run-in on the back of his mind, as United search for UEFA Champions League qualification next term.
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After joining from Paris-Saint Germain on a free in the summer there was a mass debate among fans and pundits about whether he'd have enough left in the tank to succeed in such a demanding division like the Premier League, however it didn't take long for him to prove his doubters wrong.
Zlatan Ibrahimović has racked up 28 goals in his debut campaign in England including winning goals in both the FA Community Shield and EFL Cup Final with 19 of those coming in the top flight, which has made Mourinho's men rather reliant on his services seen as though other attackers have seemed stale.
Marcos Rojo also went off with a painful-looking knock in the first half and the Manchester United boss also wasn't expecting the Argentine to make a hasty return, leaving Eric Bailly and Daley Blind the only senior centre-backs remaining to chose from as well as youth talent Axel Tuanzebe.