Lee Burge has been ruled out for around another month as he recovers from a heart issue - with Lee Johnson admitting the club is unsure whether it has been caused by Covid or by a side-effect of the vaccine.
Goalkeeper Burge was one of a number of Sunderland players who contracted Covid at the beginning of the month but, while the others have all returned to action, he has remained on the sidelines.
Johnson said last week that Burge was undergoing tests on a heart complaint, and today he went into more detail saying the 29-year-old faces several more weeks out of action although he is expected to make a full recovery.
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"He has still got an issue," he said of Burge,
"It's effectively a slightly inflamed heart and it seems to happen a lot after these injections, or potentially Covid - I'm not sure which.
"The weighting would be on Covid, but you can't rule out the vaccination.
"I don't know, I'm not a doctor.
"Burgey wasn't one that was particularly happy with how he felt in that period.
"That may have been a crossover with Covid when he had the vaccine, so we don't exactly know what has caused it.
"He's going to have to have four or five weeks where he needs to keep his heart rate below 100, and then have blood tests again to make sure his heart is fully back to normal.
"He should make a full recovery, and that's what we're expecting him to do very quickly."
Johnson also confirmed that while Burge remains out, academy goalkeeper Anthony Patterson will remain on Wearside as backup for first-choice Thorben Hoffmann.
Pattterson was recalled from his loan spell at Notts County three weeks ago and, when Burge is passed fit, is expected to go back to Meadow Lane to resume his stay and that return can happen outside the transfer window because the Magpies play in the National League.
Ahead of tomorrow's game at Bolton, Johnson says new signing Patrick Roberts is likely to be on the bench, with the ex-Manchester City winger having built up his fitness over ten days' training on Wearside.
There is also a chance that fellow new boy Jack Clarke, who joined on loan from Spurs this week, could be part of the squad.
And Alex Pritchard, who missed out on last weekend's win against Portsmouth with a calf problem has a chance of being involved, although Bailey Wright - who also missed the Pompey game with a calf injury - is not expected to travel.
Johnson said: "We're starting to get some players trickle back, obviously aside from the long-term injuries.
"Bailey Wright is out on the grass today, Alex Pritchard is back in and about it, which is very good indeed.
"Pritch has a chance of being involved tomorrow, but I wouldn't think Bailey will be although he is progressing well.
"We've looked at Patrick Roberts over 10 days now and he looks good.
"He's got a better chance of being involved this week.
"We've had to work smart with Patrick.
"I think he'll be available for us from the bench this week.
"He's got quicker feet than Michael Flatley!
"It'll be interesting to see if he can bring that to our games."
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