The 23-year-old Wales centre has not played since figuring for the Lions in the second Test against New Zealand in Wellington last summer.
Henson underwent a groin operation on his return to Wales and has been recovering ever since.
He was expected to make a return to action against Bristol in Sunday's Powergen Cup tie, although both sides cannot reach the semi-finals of the competition.
Ospreys regional rugby manager Derwyn Jones said: "Gavin suffered a slight back strain whilst weight training.
"He has not been involved in our rugby sessions for 48 hours but he is receiving intensive physiotherapy on what is a minor injury.
"We intend to give him as much time as possible to prove his fitness and we remain hopeful that he will play some part in Sunday's fixture."
Henson sat out all four of Wales' four autumn internationals including their famous 24-22 victory over Australia last weekend.
It has been a frustrating time for Henson since inspiring Wales to victory over England in the Six Nations earlier his year and then going on to win the Grand Slam.