Pete Alonso, right, meets Jeff McNeil after hitting a three-run...

Pete Alonso, right, meets Jeff McNeil after hitting a three-run home run off host San Francisco's Derek Holland in the sixth inning to give the Mets a seven-run lead on the way to an 11-4 victory Saturday, July 20, 2019.   Credit: AP/Ben Margot

SAN FRANCISCO — When you’re having the sort of historic rookie season Pete Alonso is putting together, a rare day off registers as noteworthy.

Alonso was out of the Mets’ starting lineup Saturday for the first time since May 26. Manager Mickey Callaway said the one-day reprieve stemmed from a conversation he had with Todd Frazier, a former Home Run Derby champion who said competing in the dinger contest — which Alonso won this month — can wear a hitter out.

Alonso still impacted the Mets’ 11-4 win over the Giants, crushing a three-run homer 444 feet to right-center as a pinch hitter in the sixth inning. He has 33 home runs. His 75 RBIs are a Mets rookie record, passing Darryl Strawberry’s 74 in 1983.

Alonso dubbed his partial day off as a “mental health day.”

“It went really good,” Callaway said with a laugh. “That’s what you want. One at-bat, three RBIs, homer and go rest some more.”

In the first seven games of the second half, though, Alonso went 3-for-30 (.100) with 13 strikeouts and two homers. That triggered the initial rest Sunday.

“He’s a horse,” Callaway said before the game. “But sometimes you have to take it out of their hands. Competitors don’t always want to admit things, and we understand that. And we want them to be like that.”

Alonso, adamant that he isn’t tired, understood.

“I don’t feel worn down,” he said Saturday morning. “I got to trust that my coaches and staff have my best interest at heart. I hate off days. I want to be in there because I’m really competitive and I want to be in there.

“I respect the decision.  For me, just playing and going about my business, sometimes I can’t really see the big picture and if you have an outside lens, if someone sees something, then I trust that. I trust my coaching staff.”

Callaway declined to attribute Alonso’s slow second-half start to his eventful All-Star break, calling it “probably just happenstance.”

But he and Alonso agreed that Alonso has been swinging at pitches he usually doesn’t. That, combined with some bad batted-ball luck — such as a 115-mph groundout Friday night and two near-homers last weekend in Miami — has created ugly results.

“When he’s going real well, there’s that ability to stay really quiet and take pitches and it looks like he didn’t even want to offer on that pitch at all,” Callaway said. “What I’m seeing now is obviously there’s the chase, but even the ones he’s taking, there’s a little bit more movement because he’s not recognizing the pitches as early as he used to.”

Alonso added: “I’m an aggressive hitter. I need to reel back a little bit. That’s it.”

Alonso’s NL Rookie of the Year campaign — and his pursuit of the Mets’ single-season homer record (41) — will continue Sunday, continued slump or otherwise.

“Every hitter goes through this,” Callaway said. “[Michael] Conforto’s been through it. [Robinson] Cano’s been through it. [Jeff] McNeil has not been through it. But most go through it.”

Extra bases

Tyler Bashlor needed only 15 pitches to get through two perfect innings Sunday. Callaway said it was “the best I’ve ever seen him” and “one of the best sliders I’ve seen out of anybody all year.” He added, “If he can do that, that could add another layer to our bullpen that’s already been performing pretty well.” .  .  . Zack Wheeler (right shoulder impingement) played catch and had no issues again Saturday .  .  . Mets chief operating officer Jeff Wilpon is making a rare road appearance with the team this weekend, a trip that Mets officials described as a long-planned visit that included seeing family.