Ruben Amorim has cast doubt on Tyrell Malacia's long-term future at Manchester United after claiming he lacks the attacking quality to play as a wing-back in his system.

The Dutch defender made his fifth start since returning from 18 months out with a knee injury against FCSB on Thursday, but for the fourth time he was withdrawn at half-time after a difficult evening at the National Arena.

Malacia has played left wing-back in all of those matches, but he has looked understandably rusty at times and has found it difficult to influence games in the final third, which is an area Amorim is looking to improve on with players in that position.

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The 25-year-old is likely to leave Old Trafford on loan before Monday's transfer deadline, with Real Betis one of several clubs interested in offering him the chance to build up his fitness and confidence. His opportunities will become more limited at United with the imminent arrival of 20-year-old Dane Patrick Dorgu from Lecce.

Amorim believes Malacia doesn't currently have the attributes needed to play as a wing-back, and while he hinted at a potential role in the back three, that could only happen when United become a more possession-dominant team.

"We will see," Amorim said when asked about Malacia's future. "What I know is that it’s really hard on Malacia. It is a long time without playing, and then it's hard to play a lot of games.

"A player like him is a full-back. He played with the national team outside and inside, but sometimes in this system, sometimes it's hard for him because he's not so offensive.

"He can play as one of the three defenders, if we have more possession and more control of the games. It's a really difficult context for him. But he's our player and we are going to help him."

Malacia's route to the side could also be complicated by the impending return to fitness of Luke Shaw, who trained at Carrington on Wednesday.

He didn't make the squad for the Europa League trip and won't be available to face Crystal Palace either. The 29-year-old has suffered a nightmare 12 months with injury and he hasn't started for United since February 18 last year, with just three substitute appearances under Amorim in November and December before his latest setback.

United will now ease Shaw back in and his first challenge will be to continue to work on his fitness at Carrington. His teammates trained in Bucharest on Friday morning, having stayed over in the city after their late-night Europa League victory, with the game finishing just before midnight local time.

"It will take a little bit more time," said Amorim. "I think he needs to train more, to spend more time with his teammates playing as normal, even small sided games. Not just working alone.

"He stayed [in Manchester] to do a different type of work. He's going to get together with the team when we arrive [back in Manchester]. And start from there. When I feel it is the right moment, we will start with some minutes in the games."