The Giants will likely be forced to play without their best player for the first time this season.

Mike Yastrzemski, who had appeared in each of the club’s first 49 games this year, was out of the starting lineup for Friday night’s series opener against the A’s after an MRI exam revealed a mild right calf strain.

Yastrzemski is considered day to day for now, but manager Gabe Kapler said there’s a “very low probability” that the 30-year-old slugger will be available in any capacity on Friday. Kapler said the Giants considered a stint on the injured list for Yastrzemski, but they decided to hold off to see how his body responds in the coming days, as placing him on the 10-day IL would mean losing him for the remainder of the regular season.

“I think you will always consider an IL placement,” Kapler said Friday. “At the same time, I think Yaz is important enough to this club where we really don't know how long it’s going to be, whether it’s one, two, three days or more. We want to ensure that we have some optionality there. To the degree that he’s able to play for us in the coming days, I think we have to preserve that.”

Yastrzemski departed Thursday’s 6-4 comeback win over the Mariners in the second inning after he experienced tightness in his calf during an at-bat against Nick Margevicius. He said he felt a “jolt” in his leg when he started to swing and then held up to take a pitch.

“Yesterday it was just not feeling good,” Yastrzemski said. “It didn’t feel like I was going to be able to help the team in the circumstances. I kind of decided to make the smart decision to not try and make it any worse. I’m feeling a little bit better today. It’s kind of going to be moving day to day and see how it feels and keep evaluating it daily.”

Losing Yastrzemski for any length of time will be difficult for the Giants, who have watched the late bloomer grow into a National League MVP candidate in his second season in the Majors. Yastrzemski is batting .285 with a team-high .944 OPS and nine home runs in 2020, production that will be sorely missed as the Giants attempt to make their final push for the playoffs.

“It hurts,” Yastrzemski said of the timing of his injury. “Obviously, it’s not any way that I’d like to be handling it right now. Obviously, I’d rather be playing. But if I can’t go, I can’t go, which is unfortunate. Hopefully I’ll get healthy as soon as possible. I know these guys have been working so hard, and they’re going to keep playing their tails off. I have all the confidence in the world in them to make sure that I can get another chance to play again this year.”

Darin Ruf made his second appearance and first start of the season in right field in place of Yastrzemski on Friday, with left fielder Alex Dickerson sliding into the leadoff spot against A’s right-hander Chris Bassitt. It’s a suboptimal defensive alignment for the Giants, as both corner outfielders profile as bat-first players, but Kapler said he’s hoping the club is able to take an early lead and deploy rookie Luis Alexander Basabe as a defensive replacement.

“I don’t think it’s the most comfortable outfield configuration right now,” Kapler said. “In a perfect world, we’d have Yaz out there. You could make a case that Basabe would do a nice job on defense out there. I don’t think Ruf and Dick are Gold Glove winners, but they both work really hard for this moment, to be able to step up and play solid ‘D’ for us.”

The Giants considered bringing up an outfielder from the alternate training site to bolster their defensive options, but they chose to prioritize fresh arms for their overworked bullpen, which was forced to cover 12 1/3 innings over the last two games. Shaun Anderson and Andrew Suárez were added to the 28-man roster on Friday, while Wandy Peralta and Rico Garcia were optioned to Sacramento.

Kapler admitted that it was particularly tough to send down Peralta, who came in to deliver three no-hit innings on Thursday after left-hander Tyler Anderson was ejected in the third inning for arguing balls and strikes.

“It’s very difficult,” Kapler said. “He earned a lot of respect yesterday and throughout the season, the way he’s taken the ball. He hasn’t been perfect by any stretch -- nobody has -- but he has certainly been tough and he has been enthusiastic about taking the ball in any situation, going through lefties and righties. It’s a difficult conversation, for sure.”

Worth noting

The Giants will carry infielder Daniel Robertson and right-hander Camilo Doval on their taxi squad for this weekend’s series in Oakland. Doval, a 23-year-old prospect, could be an intriguing relief option down the stretch, as he pairs a mid-to-upper-90s fastball with a sweeping slider.