Conor McLaughlin is expected to miss Sunderland's promotion run-in after undergoing a second hernia operation as the Black Cats' central defensive injury crisis goes from bad to worse.

Lee Johnson has cofirmed that on-loan Wolves man Dion Sanderson has been struggling with a back injury and, while a scan revealed no stress fracture, he will need a period of rest.

And Tom Flanagan remains sidelined with a broken bone in his foot but may be able to return at some stage with the aid of a painkilling injection, while Jordan Willis (patella tendon) and Arbenit Xhemajli (cruciate ligament) had already been ruled out for the rest of the campaign.

That leaves Johnson with only two fit senior centre-backs in the shape of Bailey Wright, who has just returned after a two-month absence with a torn calf, and Luke O'Nien, who is primarily a midfielder but has proved himself more than capable of filling in as a central defender.

Johnson has promoted Ollie Younger from the U23 side to the first-team squad and he is set to be on the bench as cover tomorrow when Sunderland look to put back-to-back defeats behind them and reignite their promotion challenge at Blackpool.

"Conor is out," said Johnson, who was forced into a late change in the midweek defeat at Wigan when McLaughlin pulled out of the game having felt discomfort in the warm-up.

"He had a hernia operation in January and now he has had another.

"It is something that often happens when you have had a hernia operation, there is a 50 percent chance that the other side will go and unfortunately that is what has happened.

"As to whether his season is over, I suppose it depends how far our season is extended - whether we are in the play-offs or not - but to make a full recovery from a hernia you are probably talking three to four weeks, so potentially it could be the end of his season."

Johnson had said after the Wigan game that Sanderson had been carrying a back problem, and while he refused to rule the 21-year-old out for the rest of the campaign, he will be missing for a period of time as he recovers.

"It's difficult news I suppose because he's not going to be right in the short term," said Johnson.

"It was best case scenario in terms of there's no fracture but there's a potential onset for a fracture with the pain.

"To be fair he's been struggling the past couple of games to get through it and he's shown his warrior spirit actually by wanting to play and actually playing.

"You may have caught him wincing on four or five occasions [at Wigan] so it got too much to manage by therapy.

"It became a scan and we looked at the scan and like I say it was best case scenario - no stress fracture which is good, but he's going to need some form of rest and recuperation period to heal and to get rid of the inflammation.

"He's not out for the season.

"Like I say it's best case scenario in that sense. Obviously he's not our player so we've got to consider the communication and open lines with Wolves but I think Dion's opinion matter hugely as well.

"I think his opinion is 'I'm very much up for the fight and I'm frustrated by this. It's a setback but it's not fatal and it's not final for the season.'

Sunderland chairman Kyril Louis-Dreyfus watches the Papa John's Trophy Final

There's never a normal day on Wearside when it comes to Sunderland. But the future could be a brighter one thanks to Kryil Louis-Dreyfus' investment in the club

On the pitch, head coach Lee Johnson and sporting director Kristjaan Speakman have been charged with guiding the club back into the Championship - and away from the League One salary cap.

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"I love him to bits he's done ever so well since he's come in and really manned-up if you like to the role that's required and he obviously gives us good speed at the back, good intelligence and good technical ability with his passing.

"We'll miss him but I suppose a timely return from Bailey Wright means we'll just about cope within that back line."

Flanagan has not featured since the EFL Trophy final at Wembley last month, and while last week Johnson was optimistic that he was nearing fitness, the latest update is less positive.

He said: "Tom has had this problem with his foot which we have discussed before, and last time I spoke about him I was quite positive about the situation.

"But I then walked back into the staff room and got quite a lot of negativity from the physio, based on the third opinion that we had.

"They are going to treat it as a bone issue with the foot, and they haven't actually decided on the course of treatment - it could be an injection, it could be time, or it could potentially be an operation.

"But I think we will probably go down the injection route to try and snuff out this problem, which has had knock-on effects with his hamstring and then hip flexor."

On-loan left-back Jake Vokins has been out of action in recent weeks after he contracted Covid, and then an ECG detected a heart murmur.

He underwent another test this week and has been given the all-clear but he now needs to rebuild his fitness.

Johnson said: "Jake has been frustrated. It has been difficult for him because he has had effectively three-and-a-half weeks doing nothing, not being able to raise his heart rate above 60 percent - a light walk at best.

"For someone who came in, had a very good game against Accrington and planted his flag in the sand in terms of performance, then to get Covid and this heart murmur - which has now been given the all-clear- was very tough.

"He has been out there [training] but it's a very slow return as he builds-up that heart rate. Effectively, he has to go through a pre-season again because after three or four weeks out, he is having to start from scratch again."