How quickly things can change.

The Mets’ relievers had performed valiantly to start the season in the absence of the face of their bullpen, Edwin Diaz.

But that only lasted so long, and on Tuesday night the unit helped sink the Mets in their 4-2 loss to the Padres at Citi Field.

The bullpen entered play Tuesday with a solid 3.96 ERA, but promptly took a step backward.

With the Mets trailing 2-1 in the top of the ninth inning, Dennis Santana was tasked with keeping the deficit as it was, but gave up a two-run homer to Xander Bogaerts.

After the Mets scored a run during a small rally in the bottom of the ninth, the two-run blast proved to be the difference between tying the score and losing.

And it raised the Mets’ bullpen ERA to 4.07, which is right around league-average as the 14th-best in MLB.

Santana did not speak to reporters following the game.

Manager Buck Showalter highlighted missed chances at the plate, though, to explain where the Mets came up short.

“We needed five runs tonight, we weren’t able to get them,” Showalter said. “We had the opportunity.”

After the Mets brought Diaz back on a record five-year, $102 million contract this past offseason, they expected to have one of MLB’s most dominant closers anchoring a new-look bullpen.

But those plans changed quickly after Diaz tore his patellar tendon while celebrating a victory with his Puerto Rico teammates during the World Baseball Classic.

Despite that, the bullpen had largely held its own and been an early season strength.

The unit tossed four scoreless innings in the Mets’ 5-0 win over the Padres in the series-opener on Monday.

David Robertson, in his first season with the Mets after signing a one-year $10 million contract, played the most important role in that regard.

Robertson has not surrendered any runs and has allowed just one hit in his five appearances this season. He has two saves and has been used in both the eighth and ninth innings as Showalter remains open to somewhat of a committee approach at closer.

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When Robertson was signed, the expectation was he would pitch in various late-inning situations.

Under that previous vision, Robertson likely could have been an option to pitch the ninth inning Tuesday.

But with Robertson thrust into a more prominent role as the bullpen’s leader for highest-leverage situations, Showalter looked elsewhere with the Mets trailing.

“Losing Edwin stunk,” Robertson told The Post. “Obviously getting the news was terrible. Nobody wanted to hear that. …. But at the same time you’ve got to move forward.There’s a lot of guys in this clubhouse who can get the job done. The team’s still going to compete and try to win the World Series.”