Mets second baseman Asdrubal Cabrera and Mets third baseman Todd...

Mets second baseman Asdrubal Cabrera and Mets third baseman Todd Frazier react after they score on a double by Mets cente fielder Juan Lagares during the eighth inning against the Nationals at Citi Field on Wednesday. Credit: Kathleen Malone-Van Dyke

There’s been a lot of good already in this Mets season — a hot start, solid team chemistry, and a bullpen that at times seemed to be untouchable. But the one thing the Mets haven’t had to do is what they managed Wednesday night.

No, the Mets haven’t had to overcome adversity because for the first three weeks of the season they just haven’t had a lot of adversity to speak of. That wasn’t the case while facing the possibility of a three-game sweep at the hands of the Nationals, with sports pundits and Twitter warriors already bemoaning the return of “the same old Mets.” And for 7 1⁄2 innings, those naysayers seemed to be right. The Mets couldn’t quite solve starter Tanner Roark, and couldn’t quite seem to overcome the psychological burden of an early deficit. Until, that is, they could.

Todd Frazier’s two-run single with the bases loaded in the eighth tied it, and pinch hitter Juan Lagares’ two-run double drove in the go-ahead runs as the Mets came from behind to defeat the Nationals, 11-5, at Citi Field. The Mets, who trailed 4-2 entering the bottom of the eighth, batted around and scored nine runs against three relievers. The crowning glory was Yoenis Cespedes’ line-drive grand slam to left, his fourth homer of the season.

“I see guys that are not selfish in there,” Mickey Callaway said. “There’s guys that come ready to play every single day, and when you get a group like that, they’re never out of it. We scored nine runs in one inning. That’s very tough to do. They just feed off of each other because they’re all pulling for each other and they’re selfless.”

The Mets led off the eighth with three singles off Ryan Madson, and after Jay Bruce fouled out, Frazier hit the tying single to center and reached second on the throw (he brought his imaginary pepper shaker with him to celebrate). Adrian Gonzalez walked to reload the bases, and one batter later, Lagares ripped a double inside first base. The next two batters walked, forcing in another run before Cespedes’ homer made it 11-4.

“It’s too early” to panic over two lost games, said Lagares, who’s hitting .407. “We [needed to] go out there and play good baseball, and we did.”

Before that, the Mets’ fate seemed to be sealed by one bad pitch and one bad play in the field.

Steven Matz allowed a three-run homer to Ryan Zimmerman in the first, and the Nationals tacked on another run in the seventh when Bruce dived for a Zimmerman line drive that should have been a single but rolled past him for a triple. Zimmerman scored on Moises Sierra’s sacrifice fly.

Roark survived a choppy fourth inning but had an otherwise capable performance, allowing two runs and two hits in seven innings, with three walks and six strikeouts. After his 33-pitch first inning, Matz righted himself, at one point striking out three straight and retiring the side in order in the second, third and fourth. But an eventful bottom of the fourth ended up costing the lefty the rest of his night. The Mets scored two and seemed primed for more when Matz’s spot came up, and Callaway pinch hit for him. But the Mets failed to score after that and still trailed 3-2.

Matz was pulled after only 74 pitches. Paul Sewald picked up where he left off, getting the side in order in the fifth and sixth for a streak of 18 batters retired dating to April 1. That streak ended in the seventh after Bruce misplayed Zimmerman’s triple.

“It’s just huge for us to avoid the sweep and go on the road and fly to Atlanta happy,” said Michael Conforto, who led off the nine-run surge with a single. “It’s good to [have these comeback wins] early. I think it’s just the personality of this team — no quit. Todd says it all the time: ‘We play nine innings for a reason.’ ”