Ole Gunnar Solskjaer says it is "not impossible" to leave Cristiano Ronaldo out of his Manchester United side.
Ronaldo scored twice on his second United debut against Newcastle in a 4-1 victory on Saturday which sees the club sit top of the Premier League table after four matches.
But the Norwegian says the Portugal international cannot be guaranteed to start in every match.
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"No, it's not impossible to leave him out," Solskjaer said. "He is 36.
"Mason (Greenwood) is 19, so it's the same. I have to manage his minutes and I have to manage a 36-year-old's minutes as well."
Raphael Varane, signed from Real Madrid, also made his United debut on Saturday. Solskjaer believes the Frenchman, alongside Ronaldo, will make everyone in the United squad raise their level.
He added: "The more quality you add to the group, the more healthy the competition (is) and (the more) leaders (there are) who they look up to.
"I think seeing Raphael and Cristiano coming has raised everyone's eyebrows because they are winners.
"They have won everything there is to win and they put demands on themselves which the young boys, the rest of the team, look at and think that is how you stay at the top. So it is only going to be good for everyone."
Solskjaer has also backed Jesse Lingard, who came off the bench to score United's fourth goal in injury-time, to continue improving after a fruitful loan spell at West Ham last season.
He said: "Jesse has had his hard times and he is a lovely boy - or man now.
"He was allowed to go out to find himself. I could have kept him because I know how good a player he is but he showed everyone with West Ham and England what kind of player he is.
"It was a special day for him, coming back and scoring a goal in front of the fans.
"He is a Man United kid. He has grown up with the Man United kit on since he was a baby, so I am so pleased for him."
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