It’s hard not to see the physical similarities between Robert Gsellman and Jacob deGrom. Sure, Gsellman is slightly wider and his entire left arm is inked right down to the wrist. But they’re both 6-4 and imposing, sporting a poof of hair that blooms from underneath a baseball cap incapable of containing it.

More importantly, on Saturday night, Gsellman was able to mimic his doppelganger in the only way that matters to the Mets: He stared down that intimidating Nationals lineup and came back out the other side.

Gsellman played the role of Bizarro deGrom well in the Mets’ 3-1 win at Citi Field. He didn’t overwhelm the Nationals but manufactured outs nonetheless. He danced into trouble and glided out of it. Eleven batters reached base in his six innings, but only one crossed the plate.

“We’ve asked a lot of some young players and they have stepped up,” Terry Collins said. “This is what we have. These guys, I think they’re caught up in it a little bit. They were in the organization last year when things were going great up here and we’ve asked them to step in, and this is their opportunity.”

If you squinted, Gsellman wasn’t just one of the replacement pitchers meant to tide the Mets over as their injured arms recover, he was a bona fide starter in his own right. Thanks to his effort, the Mets moved within a game of the second wild-card spot. Not bad considering Matt Harvey is done for the year and deGrom and Steven Matz are hurt.

Another good sign for the Mets: They got their runs from a few unlikely sources.

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With two on and two outs in the third, Tanner Roark intentionally walked Yoenis Cespedes to get to Curtis Granderson, who entered the game hitting .116 with runners in scoring position and .048 in the same situation with two outs. But he singled on a first-pitch curve to drive in two runs and give the Mets a 2-1 lead. James Loney’s two-out RBI double, his first extra-base hit since Aug. 1, made it 3-1 in the sixth.

“A lot of teams would die to be in our spot,” Loney said. “We embrace it and we enjoy it, but we know we have to play well down the stretch.”

With one out in the first, Gsellman allowed back-to-back singles by Jayson Werth and Daniel Murphy and walked Bryce Harper to load the bases. Anthony Rendon made it 1-0 with a sacrifice fly, but Wilson Ramos popped up to end the inning.

And things didn’t get much easier for Gsellman. He shimmied out of a two-on, one-out situation in the fourth and another edgy one in the sixth. In that inning, Rendon doubled and Ramos walked with none out, but he escaped with a pair of groundouts and a strikeout.

“You just take a deep breath and don’t get too ahead of yourself,” he said. “Stay back and make a good pitch and see what they do with it . . . [This is] definitely fun baseball to be part of and hopefully we can pull it out.”

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Michael Conforto helped him do that Saturday night. He had a double and made the most important defensive play of the game. With the Mets clinging to that two-run lead in the seventh, Werth on first and Murphy at bat, Conforto laid out on a fast-sinking liner to center to make the catch and prevent what could have been at least a triple.

After the game, though, Conforto’s praise was all for Gsellman, who allowed six hits and three walks in six innings. “It’s not surprising to me,” said Conforto, who played with him in the minor leagues. “What we see here is exactly what I expected to see.”

Robert Gsellman may not be Jacob deGrom, but Saturday night, he was exactly what the Mets needed.