José Mourinho said Wayne Rooney may have to play in midfield against Swansea City because of Marouane Fellaini’s suspension for head-butting Sergio Agüero in the Manchester derby and Paul Pogba’s continuing absence through injury.

Fellaini will miss three Premier League matches, so in addition to Swansea’s visit on Sunday, he is unavailable for the trips to Arsenal and Tottenham Hotspur.

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Mourinho said Pogba would not return until Thursday’s Europa League semi-final first leg at Celta Vigo because of a muscle problem. Timothy Fosu-Mensah, who came off the bench after Fellaini’s sending-off at City to operate as an auxiliary midfielder, is also ruled out of Sunday’s game by injury.

Rooney was an unused substitute at the Etihad Stadium. Although he played regularly in midfield under Mourinho’s predecessor, Louis van Gaal, he has not done so since. On taking the United job Mourinho said he would never use Rooney in midfield – “with me he will never be a No6, not even a No8”– but he may have to reverse that policy.

“Yes, he’s an option,” he said. “We don’t have [any other players there]. We have Ander Herrera and Michael Carrick and nothing else. And Tim Fosu-Mensah, like [against City], an emergency to help the team, he’ s another who we will miss, so we are in trouble but we will fight, that’s for sure.”

In January Morgan Schneiderlin, another central midfielder, was sold to Everton. Mourinho shrugged off whether that was a mistake given he now has only Carrick and Herrera fit given the current circumstances.

“We didn’t tell Morgan: ‘We want to sell you.’ Morgan said: ‘Please sell me, I want to go and play every game.’ Sometimes there is this dilemma of whether to keep a bigger squad, but if so then players don’t want to stay. Even the younger players, when they are not playing, they ask to leave.”

Mourinho has seven players out injured. In addition to Fosu-Mensah and Pogba, Zlatan Ibrahimovic and Marcos Rojo have cruciate knee injuries, Chris Smalling a less serious knee problem, Juan Mata a groin injury and Phil Jones a toe issue.

The manager said: “We arrive in an extreme situation where, I think it must be unique in football, where we have two cruciate ligaments in the same match. In my career I had one in 17 years. And now, against Anderlecht, I have two. We are very unlucky is the only thing we can say, and the other small injuries, the Pogba one, [Antonio] Valencia last week, are the injuries of fatigue, which is normal.

“Smalling and Jones were [injured] with the English national team, training. So maybe it’s a blessing, maybe it’s an opportunity to make them mentally stronger. Maybe it’s a blessing, so let’s see if they can come back. I think the first one to come back is Mata, which will give us another option in attack, but in reality what we need now is one of the two centre-backs to be back. Pogba will be back for Celta.”

As Mourinho’s only fit first-choice central defender is Eric Bailly, the 19-year-old Axel Tuanzebe may be given a chance. “In this moment he [Bailly] is the only option we have. It’s Axel, Eric and Daley [Blind]. I think Eric and Daley they played again absolutely amazingly against City. I have to speak with them and see how they are, see how they feel.”

Mourinho joked he may have to select himself. “I’m also training hard in the gym so I can also be an option,” he said. “Mourinho and Tuanzebe against Swansea.”

The manager is pleased at how he has instilled belief into the squad he inherited from Van Gaal. “The group was a nice group. Good people, good boys, committed people. So I think Mr Van Gaal left a good group of boys with very good relations between them, and that I got in my hands.

“They missed happiness, they missed trust, belief, they missed this extra bit that brings resilience, brings you fight and they have it. So the group, next 9 July, when we meet again for next season, is a strong group, so the new players, when they will arrive, will find a group more ready to go for big things.

“But the reality is that we are fighting to win the Europa League and we are fighting to try to finish fourth, so let’s go.”