Ole Gunnar Solskjaer has revealed Raphael Varane felt a problem with his hamstring during Manchester United's 2-2 draw with Atalanta in the Champions League on Tuesday.
The World Cup winner was brought off in the 38th minute with his side a goal down, with Mason Greenwood coming on as his replacement.
The Red Devils did manage to secure a point in Italy, with Cristiano Ronaldo scoring twice - one being a 91st minute effort - to cancel out strikes from Josip Illicic and Duvan Zapata.
Speaking after the game, Solskjaer admitted he could not afford to take any chances with Varane's fitness.
"He felt his hamstring. So of course you can’t take any risks, unfortunately," Solskjaer told BT Sport. "Rafa has been such a calming influence when he’s on the pitch. We lost him but that’s football."
Varane's availability to face Manchester City in what could be another potentially defining game for Solskjaer is now in major doubt. He has arrested the slide that came with the 5-0 loss to Liverpool with victory over Tottenham and the draw with Atalanta, but another heavy defeat to a Premier League title contender could pile the pressure back on the Norwegian manager again.
There was also frustration among fans over the lengthy absence of Jadon Sancho, their summer signing who only entered the fray in the 87th minute. The England international has had a slow start to the season and didn't play at all in United's win at Spurs.
Solskjaer added: "Jadon will come good, Jadon has got a great work rate and attitude and quality.
"I thought he was really sharp when he came on, the first header that he cleared for us, he was sharp on the ball. They’ll all have a big role to play and he’ll have many many years as a good player here."