ATLANTA — Dedniel Núñez was a key piece to the Mets bullpen for two seasons, but his return to the team after Tommy John surgery hasn’t gone smoothly.

Wednesday night he had an ugly seventh inning that helped send the Mets to a series loss with a 6-3 defeat against the Braves at Truist Park.

Núñez entered with the Mets behind by one run and proceeded to allow three runs — including two home runs — while retiring only one batter.

Such is the volatility of the Mets bullpen following the trade deadline deals that sent Luke Weaver, Huascar Brazobán, A.J. Minter and Brooks Raley elsewhere. In addition, Devin Williams is on the injured list with a right shoulder strain.

Big showings in Cleveland and Pittsburgh and an impressive opener at Truist Park this week offered the Mets hope that surging bats would lead to a third straight series victory. No such luck.

The Mets were shut out Monday — and before that Saturday in Pittsburgh. In their six victories on the trip, they averaged 8.3 runs. On this night, they did not place a runner in scoring position until the ninth inning. Three solo homers accounted for their runs.

Zac Thornton was respectable, allowing three earned runs on seven hits and one walk over six innings. The rookie lefty has pitched to a 2.78 ERA and completed at least six innings in five of his last seven starts.

Thornton allowed a homer to Lane Thomas with two outs in the second inning for the game’s first run. Thomas crushed a high cutter over the left field fence — the third homer allowed by Thornton over his past four starts.

The Mets deficit increased to 2-0 in the third. Thornton walked Ronald Acuña Jr. leading off the inning and Drake Baldwin singled before Matt Olson’s double-play grounder brought in the run.

Jared Young singled to right field leading off the fifth but was thrown out by Acuña trying to stretch the hit into a double. That was the closest the Mets got to placing a runner in scoring position until Carson Benge doubled in the ninth.

Acuña’s RBI single in the fifth gave the Braves a 3-0 lead, but the inning could have been worse for Thornton after he allowed a leadoff single to Austin Riley and Sean Murphy singled. Acuña was thrown out trying to reach second following Francisco Lindor’s cutoff on the hit.

Luis Torrens’ third homer of the road trip, a solo blast in the sixth, got the Mets to within 3-1. Torrens also homered twice in Cleveland last week. Overall, he’s homered five times this season.

Luis Robert Jr. homered in the seventh to slice the Mets deficit to 3-2.

Núñez entered in the bottom of the inning and surrendered consecutive homers to Mike Yastrzemski and Riley that placed the Mets in a 5-2 hole.

Núñez was charged for a third run in the inning following Acuña’s double off the fence. Baldwin singled against Jefry Yan for the run.

Bo Bichette homered in the ninth for the final run.