CHICAGO — "Bullpen By Gabe" sprang a leak.

Three outs from an uplifting come-from-behind victory that would have set them up for a possible three-game sweep of the Chicago Cubs and a .500 record on a grueling three-city road trip, the Phillies blew it in the most deflating way imaginable. Rookie sensation Seranthony Dominguez was charged with the first runs of his major-league career and lefty Adam Morgan served up a grand slam to Jason Heyward in the bottom of the ninth inning of a 7-5 stunner that left a packed house at Wrigley Field positively delirious.

Morgan tried to go down and away with a 2-2 pitch to Heyward, a lefthanded hitter who was batting .139 with a .344 OPS in 40 plate appearances against lefties this season. But Morgan wound up elevating a 97-mph fastball that hung over the plate, and in the time it took for the ball to come to a landing in the right-field bleachers, the narrative changed from Dylan Cozens lifting the Phillies to a win with his first major-league home run to another examination of manager Gabe Kapler's unconventional approach to using a bullpen.

"Definitely a tough loss," said Kapler, as despondent as he has appeared after a game all season.

But this is the consequence of not designating one reliever to pitch the ninth inning. Since May 11, when Hector Neris blew his second save in a span of six days, Kapler has eschewed the practice of using a traditional closer.

Instead, Kapler has doled out ninth-inning assignments based on matchups and situations in the game. And as the past few weeks have unfolded, his confidence level in Dominguez has risen considerably as the 23-year-old righthander has recorded out after out against some of the best hitters in the National League.

Kapler turned to Dominguez with a five-run lead in the ninth inning Tuesday night after Edubray Ramos walked the leadoff batter. Dominguez, who hadn't pitched in four days, responded by retiring Javier Baez, Albert Almora Jr. and Ian Happ on nine pitches to stretch his scoreless streak to 14 2/3 innings since making his big-league debut last month.

And Kapler went to Dominguez again in the eighth inning Wednesday night. This time, Dominguez needed 14 pitches to induce flyouts from Kris Bryant, Anthony Rizzo and Willson Contreras — the heart of the Cubs' order — to keep the game tied.

When Cozens clocked a two-run homer to give the Phillies a 5-3 lead in the top of the ninth, Kapler had no doubt which pitcher he wanted on the mound to close out the game.

"There was no hesitation on bringing Seranthony back," Kapler said. "We have a lot of trust in him. We continue to have trust in him going forward. He was really, really sharp in that first inning. In that second inning he was imperfect. But he's human. He's not going to throw scoreless innings for the rest of his life."

Asked if he was expecting to pitch the ninth inning when he walked off the mound after the eighth, Dominguez replied, "Yes." Really, though, there isn't any way of knowing how Kapler wants to employ his bullpen in the ninth inning until he makes the decision, which can be tricky in an emotionally charged, high-pressure situation such as facing the Cubs in Wrigley Field.

Dominguez began the ninth inning with a four-pitch walk of Kyle Schwarber. He struck out Baez but gave up a single to Almora.

"I felt normal, like every other game," Dominguez said through a team translator. "I just went out there to get the job done, and that was my mindset."

At that point, though, Kapler replaced his relief ace with Morgan, another curious call with righthander Luis Garcia, who has stranded 18 of 19 inherited runners this season, warming in the bullpen.

"It was the right opportunity for us to give Mo a high-leverage moment," Kapler said.

Morgan walked Happ, then got Zobrist to ground into a fielder's choice. The matchup with Heyward decidedly favored Morgan, who said he believed he was a grounder away from ending the game.

"It was the typical way that I would attack a lefty," Morgan said. "It was just a bad pitch."

That happens, of course. But relievers also appreciate having specific roles. Kapler has conceded that pitchers are trained in the minor leagues for certain situations. It isn't easy to upend traditional relief roles on the fly, as "Bullpen By Gabe" is intending to do.

"I think it was a pressure-packed stadium all night long and especially live late in the game," Kapler said. "Our guys are prepared for it. Do we need practice? Yeah, of course we need practice. Are we developing? Absolutely. But these guys are already prepared and developed for the highest pressure situations."

This time, they didn't execute. Maybe it will be a learning experience for the young relievers and their first-year manager.