NEW YORK -- When the boos finally came, Jeurys Familia walked off the Citi Field mound, head down, staring at square inch after square inch of grass. He had made this walk before on grander stages, in bigger games. This one stung nonetheless.

A critical component of the Mets’ bullpen makeover, Familia had just allowed four runs in the eighth inning of a 9-5 loss to the Cardinals on Friday, capping a four-hour plunge that also saw the Mets drop a suspended game to St. Louis, 5-4, with Edwin Díaz on the mound. The two most significant cogs of general manager Brodie Van Wagenen’s bullpen makeover both blew leads, each taking losses in games that the Mets badly wanted to have.

“Obviously, as a whole, we haven’t been very effective,” manager Mickey Callaway said.

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After falling behind on Jose Martinez’s pinch-hit three-run homer off Steven Matz in the fifth, the Mets seemed to receive the boost they needed when Matz hit a solo homer in the bottom half of the frame before Pete Alonso knocked in a run and Wilson Ramos muscled a go-ahead two-run single into right field in the seventh. Just like that, the Mets had constructed a one-run lead. But with Diaz unavailable after pitching earlier in the evening, the Mets planned to save their most consistent reliever, Seth Lugo, for the ninth. That meant navigating the eighth with Familia.

In a season of misadventures for Familia, the result was his least effective outing to date. The former closer allowed a game-tying homer to Paul DeJong on his second pitch of the night, then a go-ahead three-run homer to Dexter Fowler. Signed to a three-year, $30 million contract this winter, Familia walked off the mound lugging a 6.91 ERA.

“For any athlete, sometimes we do have bad seasons,” Familia said through an interpreter. “For me at this point, it hasn’t been going well at all this year.”

That’s a theme throughout the Mets’ bullpen, where Van Wagenen’s offseason vision has muddied. In Diaz, the new Mets GM felt he had traded for the best closer in baseball. In Familia, Van Wagenen believed he had signed a lockdown setup man. In Lugo, Justin Wilson and Robert Gsellman, he thought he had compiled the pieces necessary to transform the Mets’ bullpen -- ranked 28th in the Majors in ERA last season -- into one of the game’s best.

Yet two and a half months into the season, Diaz is scuffling, with three blown saves, four losses, a .304 opponents’ batting average and a 5.29 ERA in his last 17 appearances. Familia’s struggles have run even deeper, including a brief stint on the injured list due to shoulder soreness. Wilson has spent the majority of this year on the IL, and Gsellman could be headed there after leaving Friday’s game due to back tightness. Only Lugo has been consistently trustworthy, though he also spent time on the IL, and the Mets are reticent to use him in back-to-back games.

The rest of the Mets’ bullpen has been in flux all season. After Friday’s second loss, the Mets designated Hector Santiago for assignment, with plans to keep Tyler Bashlor in Flushing. For Callaway, the specific personnel mix hasn’t much mattered; his lack of trust in his middle-innings arms has forced him to turn continually to Diaz, Familia, Lugo and Gsellman, often with imperfect results. Friday’s loss bloated the Mets’ bullpen ERA to 5.23, 27th in the Majors.

“We’re all frustrated with these losses,” Diaz said through an interpreter. “We’re still working every day. It hasn’t come out the way we wanted, but the expectations are still really high for the rest of the season.”

The season’s hourglass, however, is slowly filling. Shortly after Familia cracked, the Braves beat the Phillies to open a 7 1/2-game advantage over the Mets, who sit at 33-36, in the NL East. The Mets remain within striking distance, but don’t appear inclined to change their manager or core player mix anytime soon. To improve, they’ll have to do so from within -- beginning with better performances from Diaz, Familia and the rest of New York’s maligned bullpen.

“Losing two games today is tough,” Matz said. “We’re all pulling on the same end of the rope. We’ve still got a lot of games up, and we’ve got to keep our heads up.”