Attrition has led to this point. After a punishing campaign littered with bumps and bruises, such hardship had become inevitable. With their postseason hopes hanging in the balance, the Mets must rely on a skeleton crew of unproven arms and a cast of fill-ins that has been cobbled together throughout their slog.

Such is the grind now facing the Mets, 5-1 losers to the Phillies on Sunday, when their problems only got worse. Before the game, Yoenis Cespedes’ bad leg tightened, as did Neil Walker’s bad back. In the first inning, Asdrubal Cabrera left the game with a flare-up of the bad left knee which had shelved him for nearly three weeks.

“That took a little air out of the balloon, that first inning,” said manager Terry Collins, who must wait until Monday to see if he has any of those three available for the start of a four-game set against the Marlins.

The Mets had won three straight and six of their last seven, pulling to within 2 1⁄2 games of the Cardinals for the second wild card spot. But they were denied in their bid for a three-game sweep of the Phillies.

“It’s certainly been an interesting year, frustrating at times,” Walker said of the Mets’ latest bout with injuries. “It’s day by day right now.”

With the Mets missing three of their biggest bats, Jay Bruce went 1-for-4 with three strikeouts, extending his slump since arriving via a deadline day trade with the Reds. He’s hitting just .176 since joining the Mets.

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“He’s really, really put a lot of heat on himself,” Collins said. “We’ve got to get him to somehow calm down.”

The only consolation came in the form of a loss by the Cardinals, meaning that the Mets did not lose any ground in the race. But to preserve their chances, they must scrounge some wins while potentially playing shorthanded against the Marlins, one of the teams they must leapfrog to win the wild card.

Sunday represented the first of three critical games started by two rookies who debuted this month and a once-hyped prospect whose struggles sent him crashing to Double-A Binghamton.

“I probably haven’t sent three guys out in a row like this — ever,” Collins said “But that’s where we’re at.”

However, righty Robert Gsellman breezed through the first six innings of his first career start in the major leagues, allowing just one run before he left under duress in the seventh.

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Gsellman surrendered three straight hits, then watched all three runners score because righty Hansel Robles provided little relief. All three inherited runners scored, part of a four-run seventh inning that the Phillies used to put away the game.

Entering in relief, Robles immediately allowed a two-run double by A.J. Ellis, intentionally walked Freddy Galvis to load the bases, and then plunked pinch-hitter Peter Bourjos on the forearm to force in another run.

“I just left pitches elevated and they took advantage of it,” said Gsellman, who was charged for four runs in six innings with five strikeouts.

However, the righty pitched well enough to convince Collins that at 84 pitches, he was fresh enough to start the seventh inning. Despite the ending, Gsellman has pitched well in his two appearances, the first coming on Tuesday when he pitched 3 2/3 innings of scoreless relief for the injured Jonathon Niese.

Said Gsellman: “What they put me through in the minors, they got me ready to come up here and pitch.”

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The Mets can only hope for the same from their next two starters — Rafael Montero on Monday and Seth Lugo on Tuesday.

Montero struggled so badly that he was demoted to Double-A Binghamton in July. He saw his first big league action in 2014, though the 25-year-old has a 4.45 ERA in 17 major league games.

But with the Mets pushing back Jacob deGrom because of concerns about fatigue, the assignment fell to Montero. He has nine big league starts, though eight of them came back in 2014.

Lugo, a rookie righthander, follows on Tuesday. The former 34th rounder has allowed three runs over 11 2⁄3 innings in his first two major league starts.

“These guys are going to pitch at the major league level,” Collins said. “There’s no better learning area than to pitch in a pennant race, and learn how to deal with the pressures and the stress of what you got to go out and do your job.”