Jose Mourinho has confirmed that midfielder Nemanja Matic picked up a "muscular" injury during Manchester United's 4-2 win over Watford on Tuesday night.
The Serbian international was forced to withdraw after 54 minutes with his side 3-0 up and seemingly in control.
Watford launched a comeback attempt, scoring twice in the latter stages, and Mourinho admitted that United missed the security of Matic in midfield.
"Matic is injured, I don't know how badly," Mourinho said, quoted by Sky Sports. "For Matic to tell me he wants to be replaced, he does not do that if it is nothing important. It is muscular.
"We lost stability when Nemanja Matic had to come out but the important thing is the three points."
United could now be without Matic and fellow midfielder Marouane Fellaini for Saturday's clash with Arsenal at the Emirates Stadium.
Ander Herrera, who has been used as a substitute in the previous three games, could start having replaced Matic against Watford.
Victory at Vicarage Road took United to within five points of league leaders and rivals Manchester City, although they were left holding on to their lead before Jesse Lingard's late strike sealed the win.
"The second half should have been five or six-nil, it was easier to score than the first. We missed chances," Mourinho said.
"Out of nothing, they got a penalty for 3-1, then it was a different game. It was an easy match to win comfortably. At 3-2 the game is in the balance then Lingard had the initiative to kill the game.
"We should be smoking cigars but then were in trouble."