BOSTON — Marcus Thornton hasn’t exactly been himself this season.

The 27-year-old reserve guard is counted on to score in bunches and provide a jolt of energy off the bench — and that’s about it. It’s a role both Thornton and Celtics coach Brad Stevens understand and recognize, and one Thornton often filled during his first preseason in Boston.

But in the month-plus since the regular season began, Thornton’s track record has been spotty at best.

He’d scored in double figures in just six of the Celtics’ 17 games entering Sunday and logged six and eight minutes of playing time, respectively, in wins over the Detroit Pistons and Los Angeles Lakers this week.

Inconsistent floor time is part of the job description for an NBA backup — especially one on a team with as deep a backcourt rotation as the Celtics — but one can point to just a few occasions this season on which Thornton utilized his to the fullest.

He finally did so Sunday. Thornton played just 17 minutes in a 101-93 win over the Washington Wizards but scored a season-high 21 points while hitting seven of his 12 shots and three of his four attempts from 3-point range.

“I’m happy for him,” Stevens said after the game. “That’s the best part about it, because I didn’t play him in the second half either of the games this week. I felt like we had a pretty good thing going with Phil (Pressey), and I didn’t play Phil at all (Sunday) because I wanted to play Marcus Smart. But Marcus Thornton really played a great game.”

Thornton scored 11 points in the second quarter as the Celtics built a 17-point halftime lead and another eight in the fourth. The Wizards clawed back to within a point in that final frame but never were able to pull ahead.

“His shots when things weren’t going our way were big,” Stevens said. “He hit a three when they had cut it inside 10. Every shot matters, and he hit a lot of big ones in just 17 minutes.”

Thornton, who averaged a full 10 minutes more per game last season with Brooklyn, credited his big afternoon to the ball-distribution skills of point guard Rajon Rondo, who finished with 14 assists as part of a triple-double.

“It’s great,” Thornton said. “When you have a pass-first mentality point guard that’s just on you — ‘Get open, I’m going to find you’ — it’s great. It’s great to play with him.”

Thumbnail photo via Winslow Townson/USA TODAY Sports Images