With the Clarets already 1-0 down, and facing a bombardment, he was replaced by David Unsworth after just ten minutes, and was later picked up by the television cameras with an ice pack on his thigh.
The injury is more serious than expected as manager Steve Cotterill has confirmed. "He's torn the tendon at the top of his thigh and he's going to be missing for about six weeks now and he'll be rescanned in ten days time. We'll then wait and see what develops from there, whether that be rest or a small operation.
"It is a blow, because with losing Michael Duff as well, we're now without two defenders who would be in my first choice starting eleven. The biggest disappointment with this injury was the early disruption in the Southampton game. He trained before the game, did the warm up and then fell foul to the injury after ten minutes. Had we known the extent of it before he wouldn't have played."
Graham Alexander is expected to lead the side in Caldwell's absence, he took the captain's arm band during the two home games last week.