NEW YORK — What was a prime opportunity to rebound from an ugly start to the season, the New York Mets’ nine-game homestand against three sub-.500 clubs only extended the nightmare for the second-most-expensive roster in the majors.

The Mets fell to the Washington Nationals 5-4 on Thursday at Citi Field to drop the three-game series and finish the homestand 3-6. They’ve lost 17 of their past 20 games and will begin May with the worst record in baseball at 10-21.

“Not good enough, obviously,” manager Carlos Mendoza said. “Not a secret. That’s not going to do it. We got to start winning series. Period.”

New York was six outs away from winning the series Thursday when setup man Luke Weaver took the mound in the eighth inning tasked with protecting a one-run lead. Instead, he hung a changeup that CJ Abrams blasted for a go-ahead two-run home run. The swing proved to be the difference after Ronny Mauricio struck out to end the game with a runner at second base, prompting boos from the small crowd still on hand for the matinee.

“The freedom with which we play day-to-day is kind of being suffocated a little bit,” Weaver said.

The Mets found some oxygen in the third inning when, after starter Freddy Peralta dug himself a 3-0 hole with a throwing error, MJ Melendez, hitting third in the lineup behind Juan Soto, swatted a game-tying three-run homer. Three innings later, Melendez dropped a sacrifice bunt to move Soto to third base. Soto then scored on Vientos’ double to give New York the lead.

The performance continued a torrid stretch for Melendez, who has been the Mets’ second-best hitter since being called up to the majors April 15 with a 1.061 OPS in 33 plate appearances. But Mendoza chose to pinch-hit Austin Slater, a right-handed hitter, for Melendez, a left-handed hitter, in the eighth inning with Soto on second again, no outs and left-hander Richard Lovelady on the mound.

It was Slater’s second appearance since signing with the Mets on Sunday. He grounded out before Vientos lined out and Tyrone Taylor flied out to strand Soto at second.

“Slater’s here to hit lefties, obviously,” Mendoza said. “Knowing that we’ve seen Lovelady against righties as well. Just wanted to take the chance there with a righty against him and try to do damage.”

The only damage inflicted was on the Mets’ record. They finished April by getting swept by the Colorado Rockies and losing two of three games to the Nationals. They will begin May in Anaheim for a three-game series against the freefalling Los Angeles Angels, losers of six straight games, knowing the hole is getting too deep.

“It’s hard for all of us,” Mendoza said. “We’re in this together. It’s not easy. But we’ve got to keep going. There’s no other choice here. We have a responsibility and we have to turn this thing around.”