Marcus Stroman’s first month with the Mets was to mediocrity what Pete Alonso’s rookie season has been to sensational.

On a night Alonso stood alone as the Mets’ single-season home run king, the team’s celebrated trade-deadline pickup from Toronto sputtered in his biggest spot yet since arriving.

With Stroman surrendering two home runs, the Mets lost their fourth straight, 5-2 to the Cubs at Citi Field to open an important three-game series. The loss dropped the Mets three games behind the Cubs in the race for the NL’s second wild card.

“There’s been plenty of people that wrote us off and are still continuing to write us off,” Alonso said. “We’re just going to continue to keep on working and do the best we possibly can and make the playoffs. From where we were to get where we are now, that’s a huge accomplishment in itself. The job is not done yet, so we’re going to have to keep grinding away.”

Alonso hit homer No. 42, a rocket to right-center, leading off the fourth inning against Yu Darvish that gave him the Mets’ single-season record. Alonso had tied the mark Saturday against the Braves. J.D. Davis homered in the ninth for the Mets’ other run Tuesday.

Stroman, in his return from hamstring discomfort that had curtailed his start against the Indians last week, pitched six innings and allowed four earned runs on six hits with seven strikeouts and two walks. The right-hander is 1-1 with a 4.91 ERA in his five starts for the Mets and is yet to record an out in the seventh inning. Addison Russell and Javier Baez each hit two-run homers to sink Stroman on this night.

“I need to be better,” Stroman said. “I thought I threw the ball pretty well, I just made bad pitches in big moments to Baez and Russell and they kind of capitalized there. It’s pretty frustrating, but I just have to adapt and change a few things and get back to the drawing board.”

The Cubs added a run against Chris Mazza in the eighth on Baez’s single for his third RBI of the game. Baez had smashed a two-run homer in the sixth against Stroman that put the Mets in a 4-1 hole. Kris Bryant doubled leading off the inning before Baez went to the opposite field, clearing the right-field fence, for the Cubs’ second homer of the game.

Michael Conforto, who started in center field over Juan Lagares to keep Todd Frazier in the lineup, couldn’t catch Victor Caratini’s line drive to left center leading off the fifth. The ball ticked off Conforto’s glove for a double before the ensuing batter, Russell, homered to give the Cubs a 2-1 lead.

Conforto missed on a diving catch on Baez’s shot to center in the second, but recovered to hit Amed Rosario, whose throw to third nailed Baez attempting to stretch the double into a triple.

“They were a couple of tough balls that were on a line,” manager Mickey Callaway said. “Conforto is going to play as hard as he can and we trust him out in center.”

Alonso unloaded on Darvish’s first pitch of the fourth inning to break the franchise record he shared with Todd Hundley and Carlos Beltran. The next big milestone for Alonso is Aaron Judge’s MLB rookie record of 52 homers in a season.

The Mets wasted Conforto’s triple leading off the second. Russell robbed Wilson Ramos of an RBI single with a leaping catch on a line drive that stuck in his webbing before Davis was retired on a fly to shallow center and Joe Panik’s shot to right field was caught, ending

the inning.

“We’ve had sky-high belief in ourselves all year long,” Alonso said. “It’s frustrating, but on the flip side we know we can play better and there’s a lot of season left and all we need to do is keep grinding it out. We’ve bonded through the struggles of this year.”

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