SAN FRANCISCO— The wear and tear from back-to-back seven-game series seems to be taking its toll on the Boston Celtics at the worst possible time. Tired legs have led to shots falling short down the stretch in back-to-back losses that have left the Celtics on the brink of elimination heading into Game 6 of the NBA Finals on Thursday night in Boston. The Celtics missed 11 of 15 shots in the fourth quarter of a 104-94 loss to the Golden State Warriors in Game 5 on Monday night. “I had a couple shots that were short,” star Jayson Tatum said.