The former Hull City centre-back, who has been absent since November with a knee injury, has suffered a major setback and pulled up in his second week of full training this week.
The 26-year-old will have scans today to get to the root of the problem and club medics are worried he may need a second operation which would end his season.
Down and out? Turner may sit out the rest of this term after breaking down in training
Manager Steve Bruce said: 'It is pointless speculating on what might or might not be wrong - we'll just wait for the results and take it from there.
'The specialist will use a keyhole camera to look inside and we're keeping our fingers crossed it's nothing serious.
'Michael is as tough a footballer as you'll find so for him to pull up and say he wasn't happy with it, tells you something. He'll play through the pain barrier every time so it's clearly a niggle which has him worried.
'It's a tricky one but it's quite possible the problem he's got isn't anything to do with the injury he sustained last year at all, but the thing that was originally troubling him in the first place.
'I feel for him and I feel for us because we've missed him an awful lot.' Wish you were here. Barbados visit prompts Bruce to call for winter break!Elmohamady aiming to secure Sunderland deal before the summerVilla ace Bent insists he won't buckle despite feeling pressure of ?24m dealAll the latest Sunderland news, features and opinion
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