There was a high level of regret for Doc Rivers upon reflection.

The Celtics coach knew he erred when he left Brandon Bass on the bench for nearly the entire fourth quarter of Game 4 against the 76ers Friday night, a Boston loss.

Bass had been erratic offensively in the Eastern Conference semifinals, but he had exhibited a toughness all season the Celtics lacked in previous years in which they had disappointed in the playoffs.

Bass is undersized, but he never has feared diving into the paint, rising for a two-handed dunk. And he never has been shy about launching his midrange jumper, a shot that he works on tirelessly, even during those days when he played in Orlando when he was banished to garbage time by just-fired Magic coach Stan Van Gundy.

Monday night in Game 5 against the Sixers, when the Celtics offense needed a boost after Philadelphia dominated the first half and threatened to take control of this stunningly competitive series, Bass produced one of the best quarters in Celtics playoff history, proving relentless and unstoppable during a critical stretch.

His 18 points in the third quarter (and 27 overall) helped the Celtics fight off a valiant 76ers team, his outburst the primary reason why Boston cruised to a 101-85 victory at TD Garden.

Bass ruled the paint in the third quarter, and the Celtics depended greatly on his production as they shook off a lethargic first half, finally gaining a semblance of momentum in the series after the Game 4 debacle.

“To be honest with you, I wasn’t really frustrated,’’ Bass said about missing all but three seconds of the fourth quarter of Game 4. “I trust Doc and his coaching ability. For me, I just stay ready, and a night like tonight I was able to help.’’