Pete Alonso sits with sore wrist before Mets fall to Atlanta on Ronald Acuna Jr's walk-off HR off Jacob Barnes

ATLANTA — Pete Alonso remained on the bench due to a sore left hand/wrist as the Mets lost to Atlanta, 5-4, on Wednesday.

X-rays were negative, but he "probably" will get an MRI, manager Luis Rojas said. The Mets believe the issue stems from Alonso getting hit by a pitch in St. Louis on May 5 and, according to Rojas, he started to feel it more early this week.

Had he been healthy, Alonso could have pinch hit in any of several prime late-inning opportunities — for Jake Hager to begin the eighth, Cameron Maybin to begin the ninth, for Jose Peraza at the end of the ninth. Instead, the Mets (20-17) had to stick with their backups — and the backups’ backups — as they missed out on a series sweep.

Before the game, Rojas said Alonso’s absence was "just a day off." After the game, Rojas acknowledged that he kept the truth a secret so that Atlanta would not know.

"It’s a competitive disadvantage when you say that," Rojas said. "We agreed on the day off today that will connect with the day off tomorrow, then reassess on Friday to see how he feels."

Ronald Acuna Jr. hit a walk-off home run against righthander Jacob Barnes as the Mets missed their chance to sweep the series.

Having already escaped an eighth-inning jam created by lefthander Aaron Loup, Barnes was tasked with a second inning of high-leverage relief because the Mets’ bullpen was depleted. Rojas’ options for the ninth were Barnes, Trevor May (save chance only) or Joey Lucchesi (9.19 ERA). The others were unavailable due to recent workloads or already appearing in the game.

The overworked bullpen was a result of Taijuan Walker (left side tightness) pitching only three innings Monday and the Mets declining to call up a starting pitcher Tuesday.

"There was definitely some guys very excited about (a day off Thursday), for sure," Barnes said.

It only took one pitch for Acuna to win the game for Atlanta (20-23). A cutter over the plate ended up over the wall in centerfield.

Lefthander David Peterson allowed three runs and five hits in 4 2/3 innings, striking out five and walking one. But the line in the boxscore doesn’t nearly begin to tell the story of his outing.

For four-plus innings, Peterson was nearly perfect, facing the minimum number of batters.

And then he melted down in the fifth. He had a runner (Austin Riley) on third with two outs when five consecutive Atlanta batters reached base. Opposing pitcher Charlie Morton hit a go-ahead single.

"He was cruising, then one inning he just lost it. Not the same pitcher," Rojas said. "Things started to get a little fast for him . . . This is a learning process for him. He’s thrown the ball really well for the majority of the innings that he’s had this year, but he’s had that one or two innings in a game when things build up"

Peterson said: "That whole time, I felt like we were one pitch away from getting out of that inning."

Morton allowed one run and a season-low two hits in six innings. Jonathan Villar hit his fourth homer, second on the team behind Alonso (six).

The Mets’ ragtag lineup included just three first-stringers and a No. 3 hitter, Maybin, who the day before was a .103 hitter for the Triple-A Iowa Cubs.

"We feel that guys from our pitching staff have stepped up and kept games close like this," Rojas said. "Tomorrow we have a needed — much-needed — day off. We hope to find everyone fresh for Friday."