Ibrahimovic, 35, fell late in the second half as he went for a lofted ball and appeared to hyperextend his right knee when landing before the match went to extra time tied 2-2 on aggregate.

Earlier in the match, Marcos Rojo was stretchered off after being hit in the knee while going in for a challenge.

Anthony Martial replaced Ibrahimovic and Marcus Rashford hit the winner for Mourinho's team in the second period of extra time to send Man United through to the semifinals of the competition by a 3-2 aggregate scoreline.

After the match, Mourinho said: "I don't think they are easy injuries, but I prefer to wait for all the tests to be done tomorrow.

"I want to wait. My feeling is not great for both. I want to be optimistic, but I'm not.

"I can't say any more than that, I am not a doctor."

Zlatan Ibrahimovic grimaces in pain after falling awkwardly in Man United's Europa League match vs. Anderlecht.

When asked if the initial prognosis was fairly positive, Mourinho added: "Fairly negative. I think."

Daley Blind replaced Rojo in the first half and Mourinho said that the Netherlands defender was a suitable deputy if the Argentina international were to miss extended time, while adding that Phil Jones and Chris Smalling are nearing returns from injuries of their own.

"We have Daley Blind [in defence] who has experience in the position and I think it is time for Jones and Smalling to be brave and to risk, because for the team you have to do everything.

"You cannot do miracles, so I don't expect miracles, but we do everything to accelerate the process.

"But even with a crazy mentality, they cannot play Sunday. But with a special spirit, it can be sooner than expected. Instead of mid-May, beginning of May.

Henrikh Mkhitaryan opened the scoring for United, but the visitors levelled things before half-time when Sofiane Hanni put a loose ball away after an Anderlecht shot from distance came crashing off the crossbar. 

Manchester United will learn their semifinal opponent in Friday's Europa League draw, with Celta Vigo, Lyon and Ajax joining the Premier League outfit in the pot.