The Wales centre has not played since the second British & Irish Lions Test in New Zealand in July but has been pencilled in for the Ospreys' European Cup tie at Leicester on Sunday.
"Gavin was devastated not to play in this game as he was really looking forward to it," said Ospreys head coach Lyn Jones.
"But it was one of those things that quite commonly happens in a rugby player's week when he is weight training and does not do his technique correctly.
"He trained on Saturday morning and looked very good but his back stiffened afterwards and he could not move freely prior to coming to Bristol.
"If he is to have any chance of playing at Leicester, he would need to train fairly injury-free and it was not worth risking him and possibly aggravating the injury for this match."