Pep Guardiola accepts Nicolas Otamendi is not happy at Manchester City because of a lack of game-time.
Otamendi is competing for selection with City captain Vincent Kompany, England defender John Stones and previous club-record signing Aymeric Laporte at centre-back.
The Argentina international has only started eight Premier League matches this term, having to take a place on the bench for a further 13 matches.
Otamendi is in line to feature when City travel to Burton Albion for the second leg of the EFL Cup semi-final on Wednesday, the Premier League champions holding a 9-0 lead from the first meeting.
And Guardiola concedes it is becoming more challenging to keep players such as Otamendi satisfied despite City still competing for four competitions this season.
"He's not happy - I don't have an answer," Guardiola told reporters at a news conference.
"When he plays he's happy, when he doesn't he's not happy. I don't know what I can do. When he doesn't play he's not happy.
"He could play every week, he can play and I know he's there. He can help us in the build up, I've got three central defenders and Vinny [Kompany] as well. He played [against] Southampton and was excellent, sometimes I have to decide one or the other.
"John came back well after the World Cup, he wants to play a lot of games. Nico - I know we can count on him, an incredible professional and always he's ready. I can understand it."
Asked if players such as Otamendi being unsettled could result in them leaving the club, Guardiola suggested it is a problem director of football Txiki Begiristain may have to handle.
"It depends on the players, I don't want sad players here," he added. "If you are not happy you knock on Txiki's door.
"At the end of the season they have to reflect. If they don't play and are not happy they have to move. I know they are able to do it and I don't have any complaints about that.
"You see the schedule, the games everyone plays. Except the last game, [Fabian] Delph three games suspended, the others play, there's not one guy with more than two or three games with not playing.
"There is not one who is not playing games and it didn't happen. Selection is the hardest part, I am honest, in my job, you look at what happens, some need extra games for the confidence, it's many reasons, not just being naive selection."
