Attrition led the Mets to this point. After a punishing campaign littered with nicks and scrapes and bumps and bruises, such hardship increasingly became inevitable.

With their postseason hopes hanging in the balance, the Mets on Sunday began a three-game stretch in which their fate would be tied to three inexperienced arms.

“I probably haven’t sent three guys out in a row like this — ever,” manager Terry Collins said before the Mets’ 5-1 loss to the Phillies. “But, that’s where we’re at.”

Robert Gsellman breezed through the first six innings of his first ever start in the major leagues. But in the seventh, he surrendered three straight hits, then watched all three runners score because righty Hansel Robles provided little relief.

The Phillies scored four runs in the frame, putting away the game and ending the Mets’ three-game winning streak. With that, the Mets needed a loss by the Cardinals later on Sunday to keep their deficit in the wild card race to 2 1⁄2 games.

The road does not get easier for the Mets, who will start former prospect Rafael Montero in Monday’s series opener against the Marlins, one of the teams they must leapfrog to seize the second wild-card spot from the Cardinals. Montero has struggled so badly that he was demoted to Double-A Binghamton.

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

Rookie Seth Lugo follows on Tuesday, when he’s slated to make his third major league start.

“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.”

The Mets may be granted some reprieve on Wednesday, with lefty Steven Matz potentially positioned to come off the disabled list with left shoulder tightness. Matz reported no issues following a throwing session on Sunday.

But plenty of work remains ahead for the Mets, who again dealt with the crushing weight of injuries. On Sunday, Yoenis Cespedes did not start because of tightness in his right quad, the same muscle that landed him on the disabled list. Neil Walker also watched from the bench because of the latest flare-up with a back issue that has plagued him for the last two weeks.

The Mets did not get through the first inning without another injury, this time when shortstop Asdrubal Cabrera was forced from the game when he reaggravated a left knee injury that recently sidelined him for three weeks.

With Cabrera out, that left just two Opening Day starters in the Mets’ lineup. And of the eight position players, half were not even in the organization at the start of the season.

Still, Gsellman gave the Mets a chance. Other than trouble in the fourth, when Jimmy Paredes ripped an RBI double, the rookie righthander appeared to be in control. Gsellman began the seventh inning with the scored tied 1-1. At 84 pitches, he had plenty left in the tank.

Collins thought so, too, sticking with the righty even as he allowed three consecutive hits. But things did not improve with intervention from the bullpen.

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

Gsellman was charged for four runs in six innings, marring what had been an effective start until the seventh. Robles was charged with a run when Ellis came around to score on a sacrifice fly.

With a skeleton lineup, the Mets missed a chance at a big inning in the first. They loaded the bases with one out against Phillies starter Vince Velasquez. But they managed only a Curtis Granderson sacrifice fly.

Jay Bruce went 1-for-4 with three strikeouts, extending his slump since arriving with the Mets via a deadline day trade with the Reds.