Simon Grayson has confirmed Paddy McNair's injury setback will see him miss the opening six weeks of the new campaign.
Sunderland face a tough opening month to life in the Championship without two key players.
McNair and Duncan Watmore are both working their way back to fitness from long-term knee injuries and McNair will be out for longer than expected after needing to go back under the surgeon's knife.
The 22-year-old, out since last November after rupturing his cruciate ligament, is understood to have needed a minor procedure on his knee cartilage.
It is a common occurrence with cruciate ligament injuries but Grayson admits McNair's prolonged absence is a blow with the former Manchester United man expected to play a prominent role this campaign.
In August, Sunderland face Derby County, Norwich City, Sheffield Wednesday, Leeds United and Barnsley in a busy opening month, with Bury away in the EFL Cup too.
McNair is also a doubt for Northern Ireland’s World Cup qualifiers against San Marino and Czech Republic in early September.
He had been on course to be fit for the start of the season and spent time in Portugal last month with Watmore working on his fitness.
Grayson said: "Paddy had a little bit of a setback with his knee and has had to have a bit of a clean up in there which probably means he will miss the first six weeks of the season.
"So, that is a blow for us. Duncan is obviously a bit behind schedule as well so will probably miss the first month - they are two key players.
"But you have seen at Bury we have some good young players. We wish Paddy a speedy recovery and Duncan to be back quickly as well.
"We like to think all the players we have on our books will be a handful for most teams in the Championship, the same goes for Paddy. You have to earn the right.
"Paddy is in good hands and will come back in a good condition. He just had a set back which can sometimes happen after a long injury.
"I have not seen him yet, with being in Austria on the training camp and then the friendly at Bury.
"He is coming up to Scotland with us so that will be the first chance I get to catch up with and speak with him."
McNair, who represented his country at Euro 2016, had made nine Premier League appearances last season after arriving from Manchester United before injury struck.
Grayson said McNair, like the rest of the Sunderland squad who have dropped down a division, will have to prove they have what it takes to compete in the Championship when the league kicks off.
The steep learning curve begins with the visit of Derby County to the Stadium of Light on Friday, August 4.
Grayson watched intently during Friday's 3-2 win over Bury and has warned the players will need to get to grips with the second tier quickly.
"The players all have reputations of being Premier League players and now they are Championship players so they have to earn the right," added Grayson.
"You see little glimpses of the game at Bury, it is a bit new to one or two of them.
"If they think this was a bit new just wait until a few weeks when it is really hairy and scary in the Championship.
"It is a learning curve for the players, the Championship but hopefully they will learn very quickly."