The Giants were reluctant to call up red-hot outfielder Luis Matos only to have him cool his heels on the bench.

When Mitch Haniger broke his right forearm Tuesday night when hit by a pitch, the process of promotion was accelerated — and led to Matos making his major league debut on Wednesday in St. Louis.

Matos, the 21-year-old who has rocketed up prospect rankings with a sensational season so far, didn’t waste a moment in his first big league at-bat out of the second spot in the order. The 5-foot-11 right-hander swung at the second pitch and roped a single in between the third baseman and shortstop.Matos clapped his hands furiously after his first-inning single, which came before family members in the stands.

“To me it was just a relief, saying to myself I got the first one,” Matos said. “There’s always that pressure of when the first hit comes. I got it, and I could let the game continue.”

He would later score a run in the first and received a noteworthy reception upon returning to the dugout. Matos was retired in his next two at-bats before being lifted in favor of pinch-hitter Michael Conforto in the seventh inning, finishing the day 1 for 3.

“I thought it was a great major league debut and then he got introduced to Giants baseball — ‘Welcome, we hit for you in your first game,’” Giants manager Gabe Kapler said with a laugh after the game.

Matos seemed to take it in stride, saying through translator Erwin Higueros, “I wasn’t surprised. I know that’s what this team does. The manager spoke to me, he told the reasoning behind it and I understand. Get somebody up there with more experience who can help the team win, and focus on that.”

If the Giants are fortunate, Matos will prove another shot of adrenaline to a team that was given a spark by two other rookies — infielder Casey Schmidt and catcher Patrick Bailey.

Haniger and J.D. Davis were removed from an 11-3 win over the Cardinals on Tuesday night after being injured on consecutive pitches in the third inning.

After Wednesday’s game, the Giants announced that Haniger’s fractured right forearm — which happened when Cardinals pitcher Jack Flaherty hit Haniger with a pitch — would require surgery, which he’ll have today in Los Angeles. His timetable for a potential return won’t be known until after the surgery.

The prognosis for Davis, who sprained his right ankle sliding into third base, was that the severity wasn’t as bad as initially feared. The Giants elected to keep him on the roster and the hope is that he will only miss a few days.

Instead, to make room for Matos and Villar — who were both removed from the lineup after the injuries to Davis and Haniger with Sacramento facing Sugar Land in Texas and flew to St. Louis on Tuesday night — the Giants optioned right-hander Keaton Winn, who had a four-inning save in his Major League debut on Tuesday night. Villar went 1 for 3 with a double on Wednesday.

But it’s hard to blame Giants fans for being most excited about the debut of Matos., who entered the season as the team’s No. 7 prospect in MLB.com’s rankings.

He was hitting .398 with seven home runs and 20 RBIs in 24 games with the Triple-A Sacramento River Cats. He also played 31 games for Double-A Richmond, hitting .304 with three home runs and 16 RBIs.

“I think it’s all right to talk about that silver lining,” Kapler said Tuesday night. “I think Matos has been swinging the bat really well. It’s an exciting moment for this organization.”

Before Haniger’s injury, Giants announcer Duane Kuiper sarcastically addressed the possibility of bringing up Matos too early during a regular appearance on KNBR.

“Call him up and let’s sit him on the bench and let’s cool him off, cause that would be really good for him,” Kuiper said sarcastically. “If you call him up he’s got to play, and he’s got to play all the time. So he’s right where he belongs, unless there’s an injury that opens up a spot, so he just has to keep doing what he’s doing.”

According to the Giants’ pregame notes, their last player who was younger than Matos to make his debut was Madison Bumgarner in 2009.

But even with that type of youth, his carried himself like a player who knows he’s exactly where he belongs.

“(He did a) really, really nice job to stay calm and deliver for us in his first major league game,” Kapler said.