PITTSBURGH — The Mets appeared capable of withstanding the loss of the man who has been their best hitter this season. They even showed the ability to overcome the return of Jason Vargas.
But the bullpen had the final word again.

Taking the field just over an hour after trading Asdrubal Cabrera, the Mets missed an opportunity to earn their first four-game win streak in more than two months, losing 5-4 to the Pirates on Friday night at PNC Park on David Freese’s walk-off single.

Rookie Tim Peterson (2-2) was unable to record an out in the bottom of the ninth, allowing singles to Josh Harrison and Gregory Polanco, then intentionally walking Elias Diaz, before Freese got the game-winning hit over the drawn-in outfield.

Rookie Taylor Bashlor pitched two innings of scoreless relief for the Mets earlier, and manager Mickey Callaway said he plans to continue to utilize the team’s young — and inexperienced — arms in high-leverage situations going forward.

“I think that they deserve the right to go out there and show us what they can do. I thought they did a good job,” Callaway said. “There’s always pressure, especially late in the game when the score is tied, or you’re up. These young guys are gonna feel pressure they haven’t felt before. This is why we want to get them in the game, so they start getting used to that, and hopefully after a little bit of time getting in there, those tough situations will start to normalize.”

Even without Cabrera, the Mets jumped out to an immediate lead, following Michael Conforto’s three-run homer off former Yankee Ivan Nova in the first inning. Conforto, who went 2-for-3 with a walk, has three home runs since the All-Star break and 14 on the season.

Vargas took the mound for the first time since June 19 — entering with an 8.60 ERA — and succeeded by simply not burying his teammates. The free-agent bust gave up a two-run home run to Freese in the second inning, but left in the fifth with the Mets still leading.

“I didn’t really know what to expect, so to go out there and feel that good was a plus,” Vargas said. “It’s kind of hard to say what you anticipate but I felt like I was really solid out there.”

Vargas allowed three hits and three walks in 4 ¹/₃ innings, while striking out three on 84 pitches (48 strikes) and was tagged with a third run soon after leaving the field.

Seth Lugo entered with one on, and soon surrendered a two-run, bases-loaded single to Freese.

“I thought [Vargas] did a good job of battling … [and] to push him any further than we did we thought would be tough,” Callaway said. “He did a good job of keeping us where we needed to be, and we brought in one of our better relievers.”

The Mets tied the game in the sixth, and had multiple chances to reclaim the lead — at least one runner reached base in every inning but the third — but the absence of the team’s RBI leader in Cabrera became more noticeable with every zero thrown onto the scoreboard.

And after three straight wins, the Mets returned to their regularly scheduled results.