NEW YORK -- Since the Orioles gave him his first save opportunity a little over a week ago, Seranthony Domínguez had looked like the answer at closer they were searching for.

Domínguez had converted three straight saves in his last three outings -- and he had a 1.74 ERA in his first 10 appearances for Baltimore. But Monday's series opener against the Mets brought the first bump in the road since Domínguez arrived from the Phillies before the Trade Deadline.

Brought into a tie game in the bottom of the ninth inning at Citi Field, Domínguez surrendered a moonshot walk-off home run to Francisco Alvarez, and the Orioles fell, 4-3, dropping them a half-game back of the idle Yankees for first place in the American League East.

"It's not good for the team, because we're fighting for first place, and that was my mistake," Domínguez said. "Tomorrow's another day."

He said he'll be ready the next time he gets the ball with the game on the line.

"I know I can do the job," he said. "I'll be better."

Earlier Monday, manager Brandon Hyde had expressed confidence in Domínguez in the closer role, which has been up in the air for Baltimore.

"I'm kind of searching for guys to get outs in the back third of the game right now," Hyde said. "He's done really well in that spot, when we kind of had an area of need to pitch in the eighth or the ninth inning, against righties or lefties."

Dominant closer Félix Bautista is still recovering from Tommy John surgery. Craig Kimbrel has been removed from the ninth-inning role due to ongoing struggles -- he was out in the bullpen before Monday's series opener working on some mechanical fixes, with Hyde saying of the veteran: "With where we are in our bullpen here, we're gonna need guys to step up, and we need him to as well."

After the Orioles went out and got Domínguez from Philadelphia, he impressed from the get-go in a setup role. That early success prompted Hyde to give him a crack at closer duties.

"We saw 99 [mph] with a really good slider right away, as soon as he got here," Hyde said before Monday's game. "So we'll continue to try to match up, but right now, when we get to the three toughest outs of the game, the way he's throwing the ball, it's a good shot he's gonna be in there."

Domínguez was, indeed, in there on Monday night, with the score knotted at 3 in the ninth after a late O's rally had pulled them even, with Ramón Urías launching a game-tying home run off Mets starter David Peterson. But he couldn't send the game to extras.

"That was tough, losing a game like that," Urías said.

Domínguez got his first batter, no problem -- he overpowered Jesse Winker with a 98 mph fastball at the top of the zone for a leadoff strikeout, exactly the heat his manager was talking about. But the right-hander then fell behind Alvarez, 3-0, missing twice with his slider and once with his fastball.

The Mets catcher sat on the same fastball that Domínguez had blown past Winker -- also 98 mph, and in nearly the same spot (just up and in to the righty Alvarez where it had been up and away to the lefty Winker) -- and Alvarez ended the game.

"I made a couple of mistakes, got behind in the count, tried to get back in the count, and he just made a good swing," Domínguez said. "It's part of the game. Nobody wants to make a mistake."

He'll shake off the loss, and his first real hiccup with his new team. So will Hyde.

"Seranthony's been really good for us," Hyde said after Monday's loss, adding that one bad at-bat, and one big swing against Domínguez, wasn't much to read into.

"He fell behind 3-0, and Alvarez was obviously looking heater. Just beat him tonight."