The former Liverpool trainee, who had returned to the starting line-up after illness, received lengthy treatment before limping off after just 15 minutes following a tackle from Jack Robinson.
And Robins head coach Sean O'Driscoll admitted the winger could join Jon Stead, Ryan Taylor and Richard Foster on the long-term injury list.
"He is not on crutches, which is good, but he cannot put weight through his knee at the minute," he told the Bristol Evening Post.
"It is a bang rather than a twist, but we will have to wait and see. We will get him X-rayed and hope there is no crack in there.
"I'm disappointed for Paul. For him to come back into the team and for us to then lose him early doors was not great."
Bristol City's next match in their battle for survival is a trip to Pride Park on Good Friday.