Denver Hume is facing up to two months out with a knee injury, Jack Ross has confirmed.
The 20-year-old broke into the first team earlier this season after impressing for the Under-23 side but hasn't played since the end of September due to a knee injury.
The promising defender was originally due to be out for six weeks with the cartilage problem but after another setback has had to go under the knife.
He is facing eight weeks out and Ross described it as a "blow", the defender having forced his way into the first team earlier this season.
"Denver picked up a knee injury at Coventry and he had a little tear in his cartilage," said Ross.
"We knew we could manage it and he could come back, it has either been worse than we thought or he has re-injured it because he was training and was due to play at Morecambe but had to pull out the day before.
"He came back and tried to train last week but we had to get to the bottom of it.
"He had surgery on Wednesday and he will be looking at eight weeks, it is a blow for him and for me as well.
"We had no other option, eight weeks and he should be back to full health."
Sunderland travel to Walsall on Saturday in the second round of the FA Cup and the Black Cats will again be without midfielders Lee Cattermole and Dylan McGeouch.
Cattermole hasn't played since the end of October with a foot injury but Ross hopes he will be back available at the end of next week.
McGeouch is also out of the FA Cup game with an ongoing muscular problem.
Ahead of the trip to the Banks's Stadium, Ross said: "We've got a couple of knocks and bruises but hopefully nothing that will rule anybody out, a couple I'm just waiting to see how they are but I'd imagine they would travel.
"Other than that I don't anticipate anybody being back available. Lee has had a period of rest, I had a good long chat with him and we hope he will build up his recovery.
"Dylan, Saturday looks like it will come too soon.
"And obviously others like Ethan [Robson] and Denver are a little bit longer term. Charlie is getting closer and Duncan obviously came back the other day.
"Josh Maja is fine, he trained on Thursday. Most of the boys that played had a lighter session but he wanted to train as he only played just under an hour on Tuesday."
On Cattermole, Ross added: "I don't want to put a definitive timescale on it because I did that before.
"I had a good chat with him on Thursday morning, we hope he might be back with us towards the end of next week.
"He has been out a few weeks now, we have to bear that in mind too.
"I am certainly hopeful over the next week or so he'll be back within the group."