PHILADELPHIA -- The Giants have graduated a wave of young prospects to the Majors this year, but none arrived with quite as much buzz as left-hander Kyle Harrison.

Viewed as the organization’s most highly touted pitching prospect since Madison Bumgarner, Harrison showed flashes of his electric stuff in his Major League debut on Tuesday night, striking out five over 3 1/3 innings in the Giants’ 4-3 walk-off loss to the Phillies at Citizens Bank Park.

Harrison, who is ranked as San Francisco’s No. 1 prospect and the No. 20 overall prospect on MLB Pipeline’s Top 100 Prospects list, gave up two runs on five hits, one walk and one hit-by-pitch, with the entirety of the damage coming on Bryce Harper’s two-run home run in the first inning.

The 22-year-old threw 65 pitches (43 strikes), leaning heavily on his elite fastball (72 percent), which topped out at 97.6 mph and generated 11 of his 14 swinging strikes. Harrison recorded each of his five strikeouts on his heater, including his first career punchout against Trea Turner in the first.

“I thought it was about as good as you could have asked for,” catcher Patrick Bailey said.

Joc Pederson’s two-run double off right-hander Taijuan Walker in the fifth gave San Francisco a 3-2 lead that held up heading into the bottom of the ninth, but Philadelphia staged a late rally against closer Camilo Doval, who hit Bryson Stott to lead off the inning and then gave up a single to Brandon Marsh that put runners on the corners with one out.

After Marsh stole second, the Giants intentionally walked Kyle Schwarber to load the bases for Turner, who delivered a game-winning two-run single that deflected off Doval’s glove and bounced past a diving Thairo Estrada at second base, saddling San Francisco with its 12th loss in its past 16 games.

At 65-61, the Giants are now four games behind the Phillies (69-57) for the top National League Wild Card spot and a half-game behind the Cubs (65-60) and the D-backs (66-61) for the third Wild Card spot. With head-to-head records the first tiebreaker in the standings, San Francisco leads the season series vs. Philadelphia, 3-2, with one game remaining between the two clubs on Wednesday.

“It would have been much more satisfying for us to come back in after a quality debut and win the baseball game,” manager Gabe Kapler said. “You fight to get to the ninth, and you feel like you have the best closer in the game out there to win the game. It’s disappointing. It’s definitely a gut-punch.”

Harrison’s performance still served as a silver lining for the Giants, who are expected to give the young lefty a chance to stick on their big league roster and join Logan Webb and Alex Cobb as the only traditional starters in the rotation.

“It was a cool experience, for sure,” Harrison said. “Nerves were there, but once I threw the first pitch, it was the same game. … Not obviously your ideal debut, but we’ll definitely build on it.”

A 2020 third-round Draft pick out of De La Salle High School in Concord, Calif., Harrison took the mound with a 1-0 lead in the bottom of the first, but he surrendered a leadoff double to Schwarber. Two outs later, Harrison hung an 83.7 mph slurve to Harper, who crushed it out to right-center field for a two-run blast.

Left-handed hitters accounted for all five of the Phillies’ hits off Harrison, an odd trend considering he held lefties to a .171 batting average at Triple-A Sacramento this year.

“I don’t think that’s a sign of things to come,” Kapler said. “I think it’s a little bit more random and the fact that he missed with some breaking balls and some good left-handed hitters put good swings on them.”

Philadelphia tested Harrison by loading the bases with two outs in the second, but he managed to escape the jam with the help of fellow rookie Wade Meckler, who made a sliding catch on Turner’s liner to center field to end the inning.

Harrison came back out for the fourth, but he was removed after giving up a one-out single to Stott. The Giants knew Harrison wouldn’t be able to provide much length, as he’s still building up after missing most of July with a right hamstring strain.

Still, San Francisco’s bullpen nearly carried the club to the finish line, with Ryan Walker, Luke Jackson, Tyler Rogers and Taylor Rogers combining for 4 2/3 shutdown relief innings before Doval blew his sixth save of the year in the ninth.

Harrison’s parents, brother, girlfriend and a few buddies made the trip to Philadelphia to watch his debut, but he’s expecting to have a much bigger crowd on hand when he makes his first start at Oracle Park, which is only 30 miles from Concord.

“I can’t wait to get back,” Harrison said. “It’s going to cost me some money, for sure.”