José Mourinho has admitted Manchester United “are in trouble” due to an injury crisis after Zlatan Ibrahimovic and Marcos Rojo suffered what are feared to be season-ending injuries in Thursday’s Europa League victory over Anderlecht.

United have been drawn to face Celta Vigo in the semi-finals of the tournament after beating Anderlecht 3-2 on aggregate. Lyon will play Ajax in the other semi. Mourinho, whose side play Burnley in the Premier League on Sunday, said: “We are in trouble. In this moment, Eric Bailly played six matches in a row, and we lost Marcos Rojo, we don’t have [Phil] Jones or [Chris] Smalling, we are in trouble and now we lose Zlatan too.

“But in attack we have options and [Wayne] Rooney is coming [back] and let’s see if [Juan] Mata can also return before the end of the season – let’s see what happens.”

Ibrahimovic is United’s top scorer with 28 goals in all competitions. He suffered what looked a serious injury to his right knee towards the end of normal time against Anderlecht. The Swede hyper-extended his right leg and looked in considerable pain as he landed on the limb awkwardly.

The 35-year-old was carried off on a stretcher just as Rojo had been on 22 minutes, the Argentinian defender also appearing to suffer a concerning knee injury. It means Mourinho has only one fit recognised centre-back, Eric Bailly. The manager could turn to Daley Blind – who replaced Rojo on the night – or Marouane Fellaini, who are auxiliary options, or one of two 19-year-olds, Axel Tuanzebe and Timothy Fosu-Mensah to play in Rojo’s stead.

With seven Premier League matches remaining Mourinho’s side are in fifth place in the table, four points behind Manchester City though they have a game in hand, and the two Manchester sides meet at the Etihad Stadium on Thursday.

Given the injuries to Ibrahimovic, Mata, Jones and Smalling, Rooney could make a return. The club captain has been nursing an ankle injury and was an unused substitute against Anderlecht.

Mourinho said: “He has a good mentality, he is a fighter. He is a strong man, even if he is not in the best condition, even if he is not in the level of sharpness that you need to play high-level football, but he has the experience and character, so he has to.

“Now we need absolutely everyone. Now I even have to think that Axel Tuanzebe, he has to go to the youth World Cup in Korea [with DR Congo] – maybe Axel has to come now, now we need everyone.”

The Portuguese did confirm two of his starting XI to face Burnley: “[Ander] Herrera plays, he didn’t play against Anderlecht. So Herrera is fresh to play, Ashley Young is fresh to play.”

Mourinho also repeated his view that Marcus Rashford, who scored the winner against Anderlecht, should not play for England Under-21s in the summer’s European Championship. “I just feel that when a player reaches a certain level, it doesn’t make a lot of sense to drop levels,” he said.

“It’s like now I am going to play him in the under-23s, to win let’s say a derby against Man City? No. Nicky Butt [academy director] doesn’t pick him to play that game. He could by age, so when you reach a certain level, and you reach that level for a long time, I don’t think it makes sense, but the power is in their hands.”