The Boro skipper limped out of the club's 1-0 defeat at Crystal Palace clutching the back of his leg, suggesting he pulled a hamstring at the weekend.
Bates will not know the full severity of the injury until he has a scan today. But if it's a tear then the centre back is expecting to face six to eight weeks on the sidelines.
With Stephen McManus still two months away from a comeback and David Wheater having joined Bolton last month, the club are left with only two fit centre backs in Seb Hines and Maxi Haas.
Jonathan Grounds replaced the injured Bates after 22 minutes at Selhurst Park, which came as a surprise to many considering Haas, Middlesbrough's new signing from Bayern Munich, was on the bench and the youngster is naturally a full back.
I thought it was a day for Groundsy rather than Haas, because he's particularly good coming out with the ball on his left foot, explained boss Tony Mowbray.
Unfortunately, Bates' replacement was involved in Palace's match-winning goal, the ball taking a wicked deflection off Grounds. His manager, however, absolved him of any blame.
He said: The nature of the goal they scored was a moment's lapse of concentration from Seb, then a huge deflection.
But we have to be good enough to score some goals and we weren't. I don't think he (Grounds) did anything wrong, it was a huge deflection.
I don't think defense was the problem. My disappointment is that we feel as if we keep our standards high every week and we've slipped up a bit.
We knew what sort of game was coming and we've done a bit of work on breaking down teams like this in training.
We weren't that good at it during the week so it is perhaps not surprising we weren't today.
I don't think they were good enough to score either, yet they did.
Boro face another testing month, with games against some of the teams challenging for promotion. And while some fans will look at the fixtures against Swansea, Millwall, Nottingham Forest and QPR with dread, Mowbray was bullish about the prospect of his young defenders facing the Championship's top teams.
I've got no fears about the way they'll apply themselves to every game, he said.
Source: Northern Echo
Source: Northern Echo