NEW YORK -- The Pirates fell apart in the eighth inning en route to a 6-3 series-opening loss to the Mets on Monday night at Citi Field.

The game was tied at 3 when New York went ahead against Pittsburgh left-hander Aroldis Chapman. It turned ugly by the time the eighth came to an end.

Chapman already recorded an out when Jeff McNeil stepped to the plate. McNeil quickly found himself in an 0-2 hole. He swung at the next pitch and missed it for strike three, but the ball got past catcher Henry Davis for a wild pitch and that put McNeil on first base.

DJ Stewart followed and reached base on a walk to put runners on first and second. McNeil and Stewart then pulled a double steal and both came home on Harrison Bader’s double to make it a two-run game.

"I knew from [playing] center field, he has really good stuff,” Bader said of Chapman, his former Yankees teammate. “That's all you kind of need to know to formulate his approach. He changed his mechanics a little bit with the way he was delivering the ball. He popped me there with that first strike.

“You learn really quickly when you're in the box what you're up against with a guy like that and the stuff he has, which for years has been tremendous. I was just trying to take a short swing to the ball and just play with what's in front of me. I'm just happy I got a pitch to handle for my team. That's all it comes down to."

Chapman then was ejected by home-plate umpire Edwin Moscoso for arguing balls and strikes. The ejection forced right-hander Roansy Contreras into the game.

Brandon Nimmo came to the plate against him and hit into a fielder’s choice, which allowed Bader to score the Mets’ sixth run of the game.

All three runs were charged to Chapman, and Pirates manager Derek Shelton didn’t have any excuses for Chapman.

“We just have to make better pitches. It kind of got away from us there,” the skipper said.

The eighth inning wasn’t the only time the Pirates lost their bearings. Left-hander Martín Pérez was dealing in the first five innings, holding New York to three hits. Andrew McCutchen and Connor Joe highlighted a three-run sixth with RBI singles for a 3-0 Bucs lead.

However, Pérez lost the magic in the bottom of the sixth, plus it didn’t help that Joe had problems defensively in right field. With the bases loaded and one out, Pérez walked Francisco Alvarez, which allowed Starling Marte to score the first run for New York.

McNeil followed and hit a shallow fly ball to Joe in right field. While Joe easily caught it, he thought about doubling up Alvarez at first base. But Joe lost his grip on the ball and it allowed Francisco Lindor to score the second run for New York and the other runners to advance.

“I came up to throw to first base and I didn’t see anyone there. I tried to pull it down. Unfortunately, [the ball] got away from me,” Joe said.

Brett Baty then scored the tying run on a double by Stewart.

“They got me with a couple of pitches up,” Pérez said. “In the sixth, I was missing my spots. I had the walk with the bases loaded. I couldn’t get a ground ball for a double play.

“We fought to the last out. The Mets played better than us. We just have to move on and be ready for tomorrow.”