Allow home runs to the first two batters of the game? Lose your second baseman to a dastardly looking leg injury? Take out your “fatigued” starter after just 58 pitches? Have to use another pitcher as a pinch hitter with the bases loaded because you’re short of players?

The Mets scoffed at all of that Wednesday night. They rallied from an injury to Neil Walker, an ineffective and short start by a possibly injured Matt Harvey and an early three-run deficit to beat the Cubs, 9-4, before 34,566 at Citi Field.

Curtis Granderson gave the Mets their first lead when he led off a five-run eighth inning with a home run into the rightfield corner. Granderson was rewarded with a curtain call after his 300th homer, his seventh of the season.

The Mets, who have won five of six, expanded the lead later in the inning when Lucas Duda — who entered the game when Walker injured his left leg running to first base — hit a three-run homer to right. T.J. Rivera added an RBI single to cap the rally.

But the biggest hit may have been a pinch-hit single by Steven Matz in the fourth. Matz was batting for Harvey with the bases loaded and the Mets trailing 4-1 because a) Harvey’s velocity was down so much that Terry Collins didn’t want him to keep pitching and b) Yoenis Cespedes was not ready to pinch hit that early in the game because he needs time to get his injured leg ready.

Matz made his manager look good by beating out a grounder to deep shortstop for an RBI single. Juan Lagares followed with a sacrifice fly to pull the Mets to within 4-3, where the score stayed until Lagares cracked an RBI triple in the seventh to tie it.

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“Obviously, you don’t want to see Harvey go out of the game for the reason that he did,” Granderson said. “Even though we were down 4-1, the sense was that we were down more than that with all the Cubs fans here cheering as loud as they were. But we continued to keep guys realizing that ‘Hey, this thing is far from over.’ Got a lot of opportunities left and Matz, our good hitter, had to come in there and beat out that hit. It was obviously a big thing for us, a big boost.”

Said Collins: “We got the guys on and I turned to (bench coach) Dickie Scott and I said, ‘Please tell me Cespedes is ready.’ And he wasn’t. So we just said, ‘Well, we’ll go with Steven Matz.’ He saved the manager tonight . . . I’ll tell you, when Steven walked up to home plate and beat the play out, there was a sense in the dugout, ‘Hey, look, we’re back in this thing.’ ”

Harvey gave up home runs to the first two batters, Anthony Rizzo and Ian Happ, and then a 467-foot two-run shot over the Shea Bridge to Kyle Schwarber in the fourth.

Harvey, whose fastball dipped below 90 miles per hour after the first inning, said he felt “fatigue” and “discomfort” in his arm, but not pain. He is scheduled to see a doctor Thursday, as is Walker.

The Mets lost Walker in the third inning when the second baseman pulled up while racing down the first-base line to try to beat out a bunt. Walker never made it to the bag as he grabbed his left leg in pain in the hamstring area and went to the ground. The Mets later announced Walker had suffered a “left leg injury” and would have an MRI Thursday.

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Just another day in Mets-land, apparently. At least this one ended with a rousing victory.