After questionable decision-making on the final possession of Game 4 on Sunday cost them a win, the Boston Celtics found themselves in tenuous position entering Tuesday night’s Game 5 against the Philadelphia 76ers. What could have been a commanding 3-1 series lead turned into a brand new series, with the two teams taking the court in Boston looking to put the other on the ropes. That mistake proved costly — the Celtics looked sluggish and absent-minded in Game 5, and suffered a 115-103 loss to fall behind 3-2 in the semifinals for the second consecutive season.