Ruben Amorim said he was forced to work with "time limits" on his players returning from injury at Manchester United as he suffered his first defeat in charge of the club in a 2-0 loss at Arsenal.

Second-half goals from Jurriën Timber and William Saliba, both from corners, sealed the win for the Gunners and inflicted former Sporting CP coach Amorim's first league defeat in over a year.

But with the United squad still hampered by injury and a lack of match fitness, Amorim said his selections continue to be affected by the fitness concerns.

Harry Maguire, Tyrell Malacia and Mason Mount all played for less than an hour at the Emirates after recently returning from injury, while summer signing Leny Yoro made his first appearance for the club after a long-term foot injury lay-off.

"It's not because I like to change all the time, but we have to have all the squad fit," Amorim said. "So for example, Harry Maguire has a time limit, Tyrell Malacia has a time limit, Mason Mount has a time limit, Leny Yoro has a real time limit, so we have to manage within games, different methodology, different way of playing, more metres to press.

"Tomorrow we have to assess and the guys that are a little bit of risk to some injury will not play, so we have to have everybody to work, to play and to improve the squad."

Despite the defeat, Amorim said he would still take positives from the performances of his players at the Emirates.

"I make the evaluation of the game regardless of the result," he said. "Like I said, after the 4-0 [win against Everton] you can take positive things, but we have to improve and you already knew that.

"But you can feel it in the stadium, especially in the end of the first half you can feel they were not comfortable, you can feel it in the stadium, so you can take that.

"You see a goal and then the momentum changes and it's really hard for us to take full control of the game but we try it."

Amorim, meanwhile, said he would give Luke Shaw all the time he needs to be fit after the England defender suffered another injury setback this week following five months out with a calf injury.

"What I can say since I arrived, I see him in the medical department working a lot in the gym, working a lot on the pitch," Amorim said. "We control the load during the games, during the trainings. He was losing weight, so he was doing everything.

"If he continues to do that behaviour, he's a top, top player and we need him. No matter how long he will need, I will be with him, we will help him to return. Not just Manchester United, but your national team needs a guy like Luke Shaw, so we are here to help him."