Sunderland defender Jenson Seelt will require an operation to correct a knee injury and faces "six to nine months out" following scans.
The Dutch centre-back, 20, was already ruled out for the remainder of the current campaign with the injury.
However, he will now miss pre-season and at least the first couple of months of 2024-25 in his recovery.
"It was the worst case scenario for Jenson really," interim boss Mike Dodds told BBC Radio Newcastle.
"I feel for Jenson because just when he was about to get his opportunity this happens, sometimes these things happen for young footballers.