In a locker room cloaked in gloom from a fresh defeat, Paul Pierce perked up when the subject turned to Brandon Bass. The Celtics captain praised the muscular forward’s performance against Oklahoma City Sunday, when Bass pulled down a career-high-tying 13 rebounds in a 91-79 loss. Bass’s performance isn’t something Pierce — or anyone on the Celtics — has lauded often this season, because on too many nights it seems as though Bass is doing little else beyond filling a uniform. But against the Thunder, Bass grabbed more rebounds in a game than he had all season, including five on the offensive end. Oklahoma City has a thick front line with Kendrick Perkins and Serge Ibaka, and Bass threw his 6-foot-8-inch frame around well, displaying energy unseen in months. “That’s the Brandon we need every night,” said Pierce, whose Celtics face the bottom-feeding, 13-50 Charlotte Bobcats here Tuesday night. On one play with a few minutes to go in the third quarter, Bass brought the Boston bench to its feet. Pierce had just launched a 3-pointer from the right wing, and Bass hung near the left corner, approaching the rim as the ball did. Perkins anchored the left block and thought he had an easy rebound, but Bass sneaked around Perkins and was suddenly at his right side. They leapt simultaneously, each stretching out an arm, and Bass ripped down the rebound.