WASHINGTON — Mickey Callaway has found an early favorite in the Mets’ bullpen, a former starter who has rewarded his manager’s faith.

Robert Gsellman pitched for the fourth time in six games Thursday, helping to close out an 8-2 win at Nationals Park. He has yet to allow a run or a walk over four innings of work, giving up just two singles while recording eight of his 12 outs by strikeout.

Before this season, Gsellman had made 29 of his 33 big league outings as a starter. That included 22 starts last year with a 5.19 ERA, but the 24-year-old righty moved to the bullpen in spring training as the Mets’ rotation stayed healthy.

“It’s been pretty easy,” Gsellman said. “[Jerry Blevins] has kind of taken me under his wing, kind of showing me the way and pitching his ideas to me. So far, everything he says has been right. I just gotta keep listening.”

Gsellman has been the first option in relief in three of his four games. The lone exception came Thursday, when he was the first to warm up as Jacob deGrom got into a bases-loaded, no-out jam in the sixth inning with the Mets holding onto a 4-2 lead. After deGrom escaped trouble, Blevins started the seventh before Gsellman came in to strike out Michael A. Taylor and end the inning.

“It’s always an honor,” Gsellman said of being first out of the bullpen. “It’s always confidence, but we have a lot of good arms in the bullpen, so they’ll call on other guys.”

The other guys have done well, too. The Mets entered Friday with a bullpen ERA of 1.16, good for second best in the majors, after recording a franchise-worst 4.82 ERA last season.

Callaway said during the opening series of the season that he wanted to manage Gsellman’s workload as he makes the transition to full-time reliever. The kid gloves began to come off on Thursday, with Gsellman making an appearance on back-to-back days for the first time in his eight-year pro career.

Gsellman has taken to his new role as it allows him to attack batters right away rather than saving up for multiple turns through an opposing lineup.

Of the 55 pitches Gsellman has thrown this season, 30 have been sinkers, per Brooks Baseball. The improved pitch has given him a weapon to work with that was lacking at times last year.

“Man, he’s got a pretty good sinker and slider,” closer Jeurys Familia said. “Coming out of the bullpen is something he’s going to have success [with].”