Boston’s Brayan Bello delivers a pitch Sunday against the Washington Nationals. Bello was victorious in his first start since April 19, lasting five innings in a 3-2 win. Steven Senne/Associated Press

BOSTON — Ceddanne Rafaela hit a two-run double after Washington center fielder Victor Robles dropped a routine fly for an error during Boston’s three-run second inning, and the Red Sox held off the Nationals 3-2 on Sunday.

Behind a good start from Brayan Bello (4-1) and scoreless work from four relievers, the Red Sox won their second straight after losing the series opener Friday.

Boston’s team ERA dropped to 2.75, the only one in the majors below 3.00. It’s the club’s lowest ERA this late in the season since 1920.

“It’s been good,” Red Sox Manager Alex Cora said of the pitching. “The reason we are where we’re at is because of the pitching.”

Eddie Rosario homered for the second consecutive day for the Nationals, a two-run shot. Robles also had a baserunning blunder that took the Nationals out a potential big inning, and lined to left with two runners on to end the seventh.

The game ended when Jacob Young was caught trying to steal second. It was challenged and confirmed on video review.

Robles dropped Vaughn Grissom’s fly, with the ball popping out of his glove when he was lining up to throw to third with runners on first and second. One out later, Rafaela’s bloop fell in down the right-field line and bounced into the stands for a ground-rule double.

Jarren Duran then scored on a wild pitch when Romy Gonzalez swung and missed. Adding to the crazy inning, plate umpire Brian O’Nora had the wrong count on Gonzalez later in the at-bat, thinking the wild pitch was a ball even though Gonzalez took a full swing on the pitch.

In the top of the next inning, Robles ran to third – with the base occupied by Riley Adams, who was standing there after a CJ Abrams single – before he was tagged out in a rundown.

“We made some mistakes,” Nationals Manager Dave Martinez said. “A dropped fly ball, ran into some outs on the bases. It might have cost us the game, you don’t know that. But when you make those kinds of mistakes, it’s definitely going to hamper the way you finish the game.”

Bello gave up two runs in five innings in his first start since going on the injured listed April 21 because of back tightness. Kenley Jansen got the final three outs for his seventh save, the 427th of his career.

“I felt very good today,” Bello said through a team translator. “I’m even more happy that I helped the team win. … I feel like I’m still not 100 percent, but for my next outing, I’ll be up there.”

MacKenzie Gore (2-4) was charged with three runs, two earned, in his 111-pitch outing over six innings. He allowed six hits, striking out nine with two walks.

Rosario’s homer went into Boston’s bullpen, a drive that Rafaela made a running, lunging attempt on as he tumbled over the wall just after it ticked off his glove.