Ole Gunnar Solskjaer understands Donny van de Beek's frustration over a lack of minutes but insists Manchester United "can't have sulkers".
Van de Beek joined United from Ajax for a fee rising to £39m last summer but has struggled to play regularly under Solskjaer.
The 24-year-old has started just two games this season - in the Carabao Cup loss to West Ham and the Champions League defeat at Young Boys - and played five minutes in the Premier League as a substitute against Newcastle.
He looked annoyed during United's Champions League win over Villarreal on Wednesday when Solskjaer decided to leave him on the bench and bring on Jesse Lingard and Fred, but the United boss says he understands his disappointment.
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"I manage a squad full of international footballers, competitive footballers who want to make a difference, want to play. They all want to be on the pitch, of course they do," Solskjaer told reporters on Friday.
"Donny was ready to come on [against Villarreal], as all the players should be. I've probably been a substitute more than anyone in this club and you have to be ready all the time.
"I've not been very happy a few times, but that's just a decision the manager makes. Alex (Telles) was coming off as a left-back and Fred did that job well.
"I understand the frustration. Every player is keen to play. That's got to be built into energy and some determination for when you get on and then show me. That is not just Donny, that is all the players."
Although Van de Beek was upset with the decision to leave him on the sidelines, Solskjaer insists the midfielder has not let his frustration impact his team-mates in a negative way.
"If we're going to be successful together, we know that we need positive energy," the Norwegian added.
"We can't have sulkers, but Donny has never affected his team-mates negatively.
"As soon as I see players affecting the team negatively that's a different scenario and I'll probably be a little bit firmer."
Van de Beek was understood to be keen on a summer loan move to Everton - the visitors to Old Trafford in the Premier League on Saturday - but said earlier this month he trusts Solskjaer and positive discussions with United ended any prospect of a premature exit.
He has at least four years left on his United contract and will be keen to regain his place in the Netherlands squad ahead of next year's World Cup, having been left out for this month's qualifiers against Latvia and Gibraltar.
Ole: Let's build on Villarreal win
Solskjaer has called on his Manchester United players to carry the momentum of Wednesday's Champions League win over Villarreal into Saturday's clash with Everton.
United had lost three of four games going into Wednesday night but came from behind to win 2-1 at Old Trafford thanks to Cristiano Ronaldo's stoppage-time strike.
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"The boost and energy you get in the group after a win like this has to be built on," Solskjaer said.
"It's a very, very quick turnaround, Wednesday night and Saturday morning, but the players are not going to get caught out by it and we need to start as we finished on Wednesday night."
Ronaldo's late strike was his fifth goal since returning to Old Trafford on Deadline Day.
"He's had a great impact on and off the pitch in how professional he is," Solskjaer said. "Of course there are the goals - five goals in five games says everything.
"At his age, he's still fit. Of course, we've got to manage him, got to make sure he can keep going with his form. He's a very good example for everyone in how he prepares, how he conducts himself.
"It's been a very, very good impact so far and it's only been a month so long may it continue."
Rashford fitness boot, Maguire still out
United got good news on the injury front on Friday as Marcus Rashford took part in full training for the first time since his shoulder surgery, but he still needs time to build up fitness.
"Marcus trained fully for the first time today with contact," Solskjaer said. "There were a few tackles flying in on him but he seemed okay, which was nice to see.
"Luke (Shaw) feels better in himself, he'd been ill but he got a shoulder injury so we need to make a decision on him.
"Harry (Maguire) is still out and Amad (Diallo) is out, but we didn't lose anybody on Wednesday night."