On a night he enjoyed one of his most productive offensive games of the season, the Spurs’ Tim Duncan spent the final 8.7 seconds of a 92-91 victory over the Mavericks in the worst possible position: Sitting on the bench, helpless to do anything but watch as Dallas’ Vince Carter launched a potential game-winner at the buzzer. Opting for a smaller, quicker defender, Spurs coach Gregg Popovich replaced Duncan with Boris Diaw, with the Mavericks down by one. “It’s always tough to sit in that position,” Duncan said. “It is what it is. He’s got a game plan, a system in those times to go smaller. If they go smaller or have a shooter in there, he likes to put someone a little more mobile in. “You’ve got to respect it. You’ve got to sit there and cross your fingers.” After missing seven of his first 10 shots Thursday, Duncan made nine of his final 10 and finished with 28 points. He was one rebound shy of his second 20-20 game of the season, finishing with 19. It was his most productive game since returning Feb.13 from a left knee contusion suffered on Feb. 2. “It’s finally starting to come back,” Duncan said. “My shot’s not there like I want it to be. Other than that, I feel great. I feel healthy. The pain is gone. I’m starting to feel like I can actually play the game.”