Performance was poor. Leads were frequently blown. Ineffectiveness reigned.

The Mets bullpen was a walking hindrance, with one caveat. The group manager Mickey Callaway hoped to be calling upon wasn’t whole.

Even with Edwin Diaz’s continued issues, the relievers have gotten better results since the All-Star break, aided by the returns of southpaws Justin Wilson and Luis Avilan. The Mets’ bullpen entered Tuesday night’s 9-2 win over the Indians with a 3.94 ERA since the break in early July, lowering its season mark by 0.46.

But now the injury problem has returned. Robert Gsellman, a key asset to that bullpen, a pitcher Callaway described as a “workhorse,” could be lost for the season. Already on the injured list for what initially was thought to be triceps tightness, an MRI on Monday revealed a partially torn right lat muscle.

“It’s very frustrating. Nobody wants to get hurt,” said Gsellman, who is 2-3 with a 4.66 ERA in 63 ²/₃ innings. “The way the team’s playing, I want to be with them. But I’m still going to be here. I’m going to work my butt off to try and get back and support the team.”

Both Callaway and Gsellman said the hope is he can return this season, though there is no timetable. For now, the 26-year-old right-hander is merely resting and rehabbing, hoping he can get back on the mound.

Initially, Walter Lockett was called up to replace Gsellman in the bullpen, but after not being used over the weekend against the Royals, he was replaced by outfielder Rajai Davis.

Callaway said he feels the Mets are in good position to fill the void left by Gsellman with the arms currently on the roster. It will be a group effort. Gsellman was often asked to pitch more than an inning, but Callaway isn’t expecting one guy to take that role.

Recently signed right-hander Brad Brach has pitched well as a Met after struggling mightily with the Cubs. In four of his five outings entering Tuesday he had not allowed a base runner and had given up just two hits and a walk over 3 ¹/₃ innings. Avilan also has been very good, having not allowed a run over 12 ¹/₃ innings since coming off the injured list July 3. And then there is Jeurys Familia, who was a mess until recently, pitching to an ugly 7.36 ERA entering August. But this month, the former closer has struck out 12 and allowed just two earned runs in nine innings, posting a 2.00 ERA.

“Brach has filled probably that role pretty good. Avilan has stepped up, he threw quite a few games in a row,” Callaway said. “I think we have the personnel that can step up and fill those innings Gsellman was throwing. I feel like somebody is going to step up and get the job done.”

The Mets bullpen has obviously pitched far better since the All-Star break, keying this surge up the wild-card standings. But now the relievers are a quality arm short. It remains to be seen whether success will follow this latest injury. It wasn’t the case during the first half of the season.