David Moyes has doubts over Marouane Fellaini, Patrice Evra, Nemanja Vidic and Chris Smalling for Manchester United's Champions League encounter with Shakhtar Donetsk at Old Trafford.
Fellaini has been troubled by a back problem and he was one of a quartet of senior players to miss training on Monday morning.
Moyes confirmed he was dealing with a "few knocks" ahead of Tuesday's game.
Asked whether those four were the players he meant, the Scot said: "Yes."
United only require a point to seal top spot in Group A, having already secured their place in the knockout stages.
However, after suffering back-to-back home Barclays Premier League defeats in the space of four days, the Red Devils are desperate for a positive result to raise their flagging morale.
"It is tough," said Moyes.
"The expectation here is to win all the games."