The Serbian midfielder had started the game but had to be replaced in the 54th minute at Vicarage Road after signalling to the bench that he could not continue.
Two goals from Ashley Young and strikes from Anthony Martial and Jesse Lingard wrapped up the three points for Mourinho’s men and moved them to within five points of leaders Manchester City.
Matic was replaced by Ander Herrera early in the second half and Mourinho seemed to suggest that the problem is likely to be a serious one, given the Serbian’s nature.
Speaking to BBC Sport after the game, Mourinho said: “We lost stability when Nemanja Matic had to come out but the important thing is the three points.
“For Matic to tell me he wants to be replaced, he does not do that if it is nothing important. It is muscular.”
Matic’s injury setback casts doubt about his availability for Manchester United’s crunch Premier League clash against Arsenal at The Emirates on Saturday evening.
His absence would come as a blow to the Red Devils, with Matic having started all 14 of Manchester United’s Premier League games this season since his move from Chelsea in the summer transfer window.
Arsenal take on Huddersfield Town at The Emirates on Wednesday night.
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