Manchester United fear Mason Mount could be out for longer than first expected with the hamstring injury he picked up against Tottenham.
Initial tests indicated the £55million signing from Chelsea faced three weeks out and would be back just after the international break.
There are concerns, however, that he could be out for up to six weeks which would see him miss the visit of Brighton after the international break and potentially United's first Champions League group game as well as the next two matches against Nottingham Forest and Arsenal.