Sunderland striker Jordan Cook will be out for several months after damaging cruciate ligaments in his right knee.
The youngster sustained the injury on Tuesday evening during the reserves' 1-0 defeat to Burnley.
After landing awkwardly on the half-hour mark, he was stretchered off and underwent X-rays to confirm the damage.
The club's medical team will continue to assess Cook's injury before and after surgery which will keep him out for most of the season.
The news is a blow to Sunderland boss Steve Bruce who has been encouraged by Cook's impressive displays during a short loan spell with Darlington.