SAN FRANCISCO -- Last August, the Giants flew Shaun Anderson out to New York and placed him on the taxi squad in case they needed a starter for their series against the Mets.

Anderson, who is ranked the club's No. 4 prospect by MLB Pipeline, suddenly found himself on the brink of the Majors, but his long-anticipated debut was ultimately delayed after the Giants decided not to add him to their roster.

On Monday, Anderson received another call from the Giants, but this time it was official: He will make his Major League debut by starting against the Blue Jays on Wednesday -- well, barring inclement weather.

"I heard there's some rain coming," Anderson said, smiling. "But I'll hope for the best. It feels pretty good that there's an exact date to it. I'm looking forward to that."

Anderson's arrival is part of a larger shuffle of the Giants' rotation, which is currently in flux following Dereck Rodriguez's demotion to the Minors, Derek Holland's move to the bullpen and Drew Pomeranz's injury.

"As you've seen, we've shaken up the rotation a little bit, so we need somebody for that spot," manager Bruce Bochy said. "He's throwing as well as anybody down there, so that's why he's here."

A third-round Draft pick of the Red Sox in 2016, Anderson came over to the Giants as part of the Eduardo Nunez trade in 2017. The 24-year-old right-hander has logged a 4.11 ERA over 35 innings in seven starts for Triple-A Sacramento this year. Anderson has allowed only three home runs while walking 11 and striking out 37 in the hitter-friendly Pacific Coast League.

While offense has surged at the Triple-A level following the switch from the standard Minor League balls to the Major League balls, Anderson said he's managed to limit the long ball by keeping the ball down.

"I know the ball flies in those parks, and especially in that area, but I've been working on keeping the ball down in the zone and getting quick groundouts," Anderson said. "That's helped me avoid the lift on some of the guys who are trying to hit it out of the park."

Atherr arrives

Outfielder Aaron Altherr, who was claimed off waivers from the Phillies last week, was activated Tuesday and was available off the bench against the Blue Jays. The Giants cleared a spot for Altherr on the 25-man roster by designating catcher Erik Kratz for assignment Monday.

"I'm just glad for this opportunity," Altherr said. "I'm excited to be able to be over here and try to help these guys win and do some damage. I'm just excited for a fresh start and a brand new opportunity."

The Phillies cut Altherr after his 1-for-29 start to the season, but the Giants still see upside in the 28-year-old outfielder, who is only two years removed from launching 19 home runs and posting an .856 OPS over 107 games. Altherr struggled to replicate that success in Philadelphia last year, batting .181 with a .628 OPS.

"It's kind of hard to explain," Altherr said of the regression. "Last year, I just kind of started off real slow. I couldn't really get out of it. Once it started off slow, I started getting less and less consistent at-bats, and it just kind of snowballed from there. There are no excuses. I'm just hoping to come here and find that same swing I had in '17 and just try to build from there."

Altherr became the fifth outfielder on the Giants' roster, joining Kevin Pillar, Steven Duggar, Mac Williamson and Tyler Austin. Altherr can play all three outfield spots, but Bochy told him that most of his opportunities will likely come in left field.

Injury updates

• Catcher Buster Posey (concussion) caught Pomeranz's warmup session Tuesday and hit against the Giants left-hander in a simulated game. Posey still needs to run the bases, but the Giants are optimistic he'll be cleared to return to action Friday in Arizona.

• Pomeranz (left lat strain) threw 60 pitches over four innings in a simulated game at Oracle Park on Tuesday. Pomeranz is eligible to come off the injured list Friday, though the Giants have not yet announced when he'll slot back into the rotation.

"I don't know exactly what day he's going to pitch, but he passed all the tests today," Bochy said. "He's doing well."