Manchester United must wait to discover how long Phil Jones will be out for, but hope Radamel Falcao will be available again for Sunday's Manchester derby.
Defender Jones and striker Falcao both missed the 1-1 Old Trafford draw against Chelsea, and England international Jones, who is suffering from shin splints, is a particular concern to manager Louis van Gaal.
The former Blackburn player has only made one appearance since recovering from a hamstring injury that ruled him out for a month.
Jones, who has struggled to stay fit during his three years at Old Trafford, was expected to start against Chelsea before being hurt in training.
Van Gaal told MUTV: "The match against West Brom was his first after a long injury period. We brought him back and he trained for more than a week with the first team, then had to play 90 minutes.
"Then he had a sore feeling in his shin. On Saturday he could not continue training, even though it was a very light session before the match. He had shin splints, which is very painful. I don't know how long it will take [for him to recover]."
Jones's absence means United are without three injured centre-backs, with neither Jonny Evans nor 19-year-old Paddy McNair fit to face Chelsea, although both are expected to return to contention soon.
Chris Smalling replaced Jones in the side against the league leaders as he and Marcos Rojo became United's seventh central-defensive combination of the season.
Forward Falcao, who had only been used as a substitute in the 2-2 draw against West Brom, had been expected to miss the Chelsea game.
Van Gaal suggested the on-loan Monaco player, who has scored only one goal in his five appearances for United, could be fit for the game against Manchester City at the Etihad Stadium.
He added: "Falcao took a kick in training on Thursday and couldn't play because of that. We'll have to wait and see, but I don't think the injury is too bad -- today's match came too soon for him."
Meanwhile young striker James Wilson revealed that Van Gaal had persuaded him to change his shirt number from 47 to 49 this season.
He told MUTV: "As I'm a striker, the gaffer wanted me to have a nine in my number, and 49 was the closest."