Mason Mount ruled out of Manchester United's next two games
United revealed that Mount had sustained the "small complaint" during Saturday's 2-0 defeat at Tottenham.
The England midfielder is expected to be "out until the international break next month", United said.
Mount moved to Old Trafford from Chelsea in the close season for £60 million.
After winning the Champions League in 2021, Mount struggling to find his peak form towards the end of his Chelsea career.
He also delivered below-par performances in both of United's Premier League games this season, raising concerns about how he would fit into boss Erik ten Hag's plans.
Mount is not the only United close-season signing to be hit with injury issues already this term.
Danish striker Rasmus Hojlund, signed from Serie A Atalanta, has not featured for United due to a back injury.
Ten Hag's men host Forest at Old Trafford on Saturday before travelling to Arsenal the following weekend in their final fixture before the international break.