Leeds United midfielder Kalvin Phillips is understood to be facing a lengthy spell on the sidelines.
England international Phillips suffered a hamstring injury in the 2-2 draw with Brentford on Sunday and was replaced after 55 minutes.
The full extent of the damage is still to be determined but there are fears the 26-year-old could be set for an extended absence.
Phillips missed two games in October with a calf problem.
He is one of Leeds' most important players and it would be a major blow for coach Marcelo Bielsa not to have him available, with games against Chelsea, Manchester City, Arsenal and Liverpool coming up.
Leeds are still to reveal the extent of the hamstring injury that forced captain Liam Cooper off against Brentford.
Meanwhile, Patrick Bamford appeared to suffer a problem after he scored Leeds' injury-time equaliser against Brentford at Elland Road.
The striker was returning after nearly three months out with an ankle injury.