PHILADELPHIA -- The Giants are calling on their No. 1 prospect to give their pitching staff a boost in the team’s playoff push.

San Francisco will call up southpaw Kyle Harrison, who just celebrated his 22nd birthday, to make his debut on Tuesday in Philadelphia, Giants manager Gabe Kapler said on Sunday.

• What to expect from Giants top prospect Harrison in bigs

Harrison -- the No. 20 overall prospect on MLB Pipeline's Top 100 Prospects list and the No. 1 left-handed pitching prospect -- was selected in the third round of the 2020 MLB Draft and was considered the best healthy prep left-hander in that class. He’s the organization’s most hyped pitching prospect since four-time All-Star Madison Bumgarner.

“The last couple of outings, I think he's taken some real steps forward,” Kapler said. “We think he's ready to come up and help us win baseball games. That's the most important thing.”

Harrison, who will wear No. 45, flew into Philadelphia on Sunday night. He said he expected to have his parents, brother, girlfriend and a few friends in attendance for his debut on Tuesday.

“Pretty surreal,” Harrison said prior to the series opener at Citizens Bank Park on Monday night. “I don’t think it’s even hit me yet. It’ll be crazy. I couldn’t be happier to be here and ready to help these guys out for whatever they need.”

Harrison has made 20 starts this season for Triple-A Sacramento, recording a 4.66 ERA with 105 strikeouts to 48 walks over 65 2/3 innings. He has a 35.6 percent strikeout rate with a 16.3 percent walk rate this year with the River Cats. Harrison impressed in his two most recent starts, logging 11 strikeouts to zero walks across 7 1/3 innings, including a 60-pitch effort in his last start on Tuesday.

Spotty command hampered Harrison earlier in the season, but he said his month-long rehab from a right hamstring strain helped him reset and focus on filling up the zone once he returned to the mound.  

“I think it was just kind of a mental thing with me,” Harrison said. “Just slowing the game down. I think when I got hurt, rehab in [Arizona] really allowed me to slow everything down and stop worrying about things you can’t control.”

Harrison has shown the ability to cause swings and misses with his three-pitch mix of a four-seam fastball, a slider and a changeup. Here’s MLB Pipeline’s assessment of the left-hander: “Harrison's low slot creates good angle and shape on his pitches. He's still learning to harness his enhanced stuff, though his ability to miss bats in the strike zone with all three of his offerings means he doesn't need to locate them with precision. Once he refines his control and command, he'll be ready to help the Giants and should headline their rotation in the near future.”

San Francisco has Logan Webb and Alex Cobb as its frontline starters, but various injuries have caused the club to rely heavily on openers, and Kapler has been creative with his bullpen usage. Other than Cobb and Webb, no Giants pitcher has made more than 11 starts this season, so Harrison's arrival gives them another arm with starter potential.

Harrison isn’t fully built up, and he has completed five innings only once in 21 Minor League starts this year, so it’s unclear how much length he’ll be able to provide on Tuesday. Still, the Giants have several multi-inning relievers who should be able to help cover innings behind Harrison, including fellow lefty Sean Manaea.

“Obviously, we've had quite a few guys making debuts this year, and I think that always just brings a little jolt of energy to the clubhouse,” relief pitcher Tristan Beck said. “It's always fun seeing guys debut, seeing guys check off the list of firsts for strikeout, first win, all that kind of stuff. So Harrison's great. I know he's really well-liked in the organization, and I think a lot of guys are really fired up to get him up here finally after a long wait.”

With Ross Stripling landing on the injured list with a mid-back strain on Saturday, Harrison could be a candidate to stick in the Giants’ rotation, though facing the Phillies on the road with postseason implications is still a leap into the fire.

“Sometimes those nerves can give you a little bit of extra adrenaline,” Kapler said. “If you channel those nerves and that adrenaline, it can turn into velocity, it can turn into command. I think there’s a lot of poise and a lot of preparation leading up to tomorrow. I think [Harrison’s] going to handle the pressure well.”