A.J. Pierzynski, Atlanta's 38-year-old catcher, tripled, doubled and singled. He scored standing up on Pedro Ciriaco's tiebreaking sacrifice fly in the sixth inning, and his left leg appeared to hit d'Arnaud's left arm. The Mets said d'Arnaud, who left the game, is day to day with a hyperextended left elbow.
D'Arnaud, who homered and hit a two-run single earlier in the game, returned 10 days earlier from the disabled list after missing seven weeks with a broken right hand.
"We'll probably have him tomorrow if we need him," said Mets manager Terry Collins, who was grateful the injury was not more serious.
"I thought he could have dislocated his shoulder," he said. "The hyperextended elbow, I know it hurts but we've got tape. Those things you can take care of."
D'Arnaud said he was relieved he won't be returning to the disabled list.
"A lot of negative thoughts were going through my head," d'Arnaud said. "I'm just very fortunate that it's not as serious as I was thinking."
Simmons, who has six hits in the first two games of the series, was impressed by Pierzynski's busy night.
"He felt fast today," said Simmons of Pierzynski. "He didn't want to stop running. Stretching a double, going home from third on a shallow flyball, he did it all."
Williams Perez (4-0) allowed four runs on five hits in six innings. He improved after giving up two runs in the first, but manager Fredi Gonzalez said, "he wasn't the same Perez. He was erratic."
Jason Grilli pitched around Wilmer Flores' two-out single in the ninth for his 20th save in 22 chances.
Jack Leathersich (0-1) lost in relief.
Mets rookie Noah Syndergaard struggled again on the road, allowing four runs in four innings. He has a 6.52 ERA in four starts away from Citi Field.
The Mets took a 2-0 lead on d'Arnaud's bases-loaded single.
Pierzynski tripled into the right-field corner and scored on Simmons' single in the second.
Jace Peterson walked to lead off the third. He stole, and advanced to third when no one covered second base for d'Arnaud's throw, which sailed into center field. Peterson scored on a groundout by Cameron Maybin.
A large group of Mets fans, most wearing orange "Mets fans have more fun" T-shirts, took over four sections of the left-field seats.
The fans chanted throughout the game, including a "Why so quiet?" inquiry of Braves fans in the fifth inning.
Mets: RHP Jeurys Familia (left groin tightness) was held out. He felt more discomfort on Saturday, a day after being injured. "The doctor says it's not a major thing, that he still had great strength," Collins said.
Braves: 1B Freddie Freeman (sprained left wrist) did not play for the third straight game. Gonzalez said there was not enough improvement for Freeman to start. Because the Braves are off on Monday, Freeman may be held out again Sunday to give him five days for the wrist to heal.
Mets: RHP Matt Harvey will look for his first win at Turner Field since June 18, 2013, when the Mets and Braves finish the three-game series on Sunday. Harvey threw seven scoreless innings in his last start, a 3-2 win over Toronto.
Braves: RHP Julio Teheran, 0-2 in his last five starts, will try to snap his winless streak and improve his troubling 5.07 ERA. He gave up 13 hits and six runs in a 9-4 loss to Boston in his last start. ---