PITTSBURGH -- Jacob Stallings was near-speechless, clearly overwhelmed after an unforgettable night in what's been a forgettable season for the Pittsburgh Pirates.

On Fireworks Night at PNC Park, Stallings, the veteran catcher, started the festivities a little earlier than expected, launching a grand slam in the bottom of the ninth inning off closer Edwin Diaz, as the home team rallied for a 9-7 victory over the stunned New York Mets Saturday.

"Diaz has a great fastball, I was trying to be on time for it, I wasn't necessarily sitting on it. But just trying to be on time," Stallings said, shaking his head during his postgame, on-field interview broadcast on the stadium's PA system. "I didn't get it all, but I just hoped I got enough of it.

"Thankfully, I did."

Trailing 6-0, the last-place Pirates scored all their runs in the final two innings -- five in the eighth and four in the ninth -- en route to the victory. Along the way, Stallings notched his sixth career game-ending RBI with the third walk-off slam at PNC Park, which opened in 2001.

"We've got a bunch of guys who never quit and play really hard," he said. "We just hung on till the end. And we came away with it, so that's pretty cool."

J.D. Davis hit a pair of two-run homers for New York in his first start since coming off the 60-day injured list, and rookie starter Tylor Megill pitched six scoreless innings.

But it wasn't enough on a miserable day for the Mets, who detailed injuries to star shortstop Francisco Lindor and ace Jacob deGrom before the game.

Lindor is out indefinitely with a strained oblique, and deGrom is sidelined with forearm tightness in the latest round of health setbacks for the NL East leaders.

But by night's end, Stallings was the main headliner on either side. His homer was the third walk-off grand slam this season in the majors, as the catcher joined Seattle's Shed Long and Houston's Jose Altuve in that bracket.

And before the Pirates' rally, MLB teams were 314-0 this season when leading by six or more runs in the eighth inning or later.

"I couldn't believe it when it cleared the fence," Diaz said. "I thought it was a fly ball."

He wasn't the only Mets reliever to falter against the Pirates. Seth Lugo pitched just 2/3 of an inning, allowing five earned runs, three hits and two walks, seeing his ERA shoot up to 5.09.

"Our clubhouse, we're mentally stronger than that. It's not gonna get us down," Lugo said when asked if this type of loss could spring a downward turn for the Mets. "One loss doesn't define the team. We're a good ballclub and we're gonna show it [Sunday]."

Clay Holmes (1-1) recorded the win despite giving up a solo home run to Brandon Nimmo in the top of the ninth that increased New York's lead to 7-5. In the bottom half of that frame, with that same score, the Pirates loaded the bases with one out as Ke'Bryan Hayes was hit by a pitch, Bryan Reynolds walked and John Nogowski hit an infield single.

Diaz (3-4) appeared on his way to escaping the jam when he struck out Gregory Polanco looking on a full-count pitch. But then Stallings pulled his slam down the left-field line beyond the outstretched glove of Kevin Pillar, who climbed over the wall in a desperate attempt to make the catch.

Diaz had not allowed a home run in 48 1/3 innings dating to last season, the longest active streak in the majors.

"This kind of loss is ... yeah," Davis said, shrugging his head in disbelief. "It is what it is."

The Associated Press contributed to this report.