Elliot Embleton was taken to hospital in an ambulance with what Tony Mowbray fears is a "bad injury" after being stretchered off in the 1-1 draw with Hull City.
Embleton was sent off as he was stretchered off, suffering an injury in a challenge with Ryan Woods just before the hour mark at the MKM Stadium.
Mowbray has watched the incident back on several occasions and believes referee Gavin Ward made the wrong decision to show the red card. But the Sunderland boss was more concerned about the player's condition.
"He's gone to hospital, he was taken in an ambulance in a lot of discomfort," said Mowbray.
"I don't want to comment on what we think it is. Our club doctor is here and thought it was serious enough to go to hospital.
"He was in a lot of pain, hopefully whatever he needs doing he'll get looked after well. It looks like a bad injury."
On the red card decision, Mowbray said: "I've watched it back. I think the referee will be disappointed.
"I think the ball is bouncing, Elliot catches him with his knee in the player's arm. He does flick his foot but there's no contact as he's falling backwards. The contact was in the player's arm with his knee. It happens fast and maybe the referee seeing the player rolling around and holding his head made him make that decision."
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Mowbray was dealt a further injury blow as Danny Batth hobbled off early on with a calf problem, an issue that also caused Alex Pritchard to miss the 1-1 draw at the MK Stadium.
Mowbray said: "Danny felt his calf, whether it was a tear I'm not sure, he pushed off and felt a sharp pain. Elliot has got a bad one, let's wait and see the true diagnosis."
Ross Stewart returned to action with a goal from the bench to put 10-man Sunderland 1-0 up before Ozan Tufan levelled to earn a point for the home side.
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