Elliot Embleton has suffered an injury setback in training which will delay his comeback.
Embleton returned to full training with Sunderland's first team at the Academy of Light this week, four months after suffering a serious hamstring injury which required surgery.
But Black Cats manager Phil Parkinson has revealed that the 20-year-old suffered a fresh problem this week which means Embleton's hopes of a first-team return have been put on hold.
"Elliot has had a bit of a setback which is a real blow for us - it's the one downside of the week," said Parkinson.
"He trained on Tuesday and just felt his hamstring. The physio department don't thing it is where the operation was, but whether it is a bit of scar tissue or something we are not sure.
"We have to make sure that we protect him and don't push him too quickly, too soon."
Embleton spent a successful loan spell with League Two Grimsby Town last season and was expected to be involved in the Black Cats' League One campaign this term.
But he made just a handful of appearances before suffering the initial hamstring injury in Sunderland's Carabao Cup win against Sheffield United at Bramall Lane in late September, just as he was beginning to hit form.
Parkinson rates Embleton highly and was looking forward to utilising him in Sunderland's promotion challenge.
He said: "He could be a player who could be a bonus for us in the second part of the season because I have had the opportunity to watch the DVDs of his games at Grimsby last season and here this season, and I don't think anyone can dispute that he has real quality."