Jon Heyman of FanRag Sports first reported the news.
Sandoval posted a .212/.269/.354 triple-slash line with four home runs across 32 games with the Red Sox in 2017 before getting designated for assignment last week.
He struggled to reach expectations with the Red Sox after signing a monster contract ahead of the 2015 season. It's a deal that carried a base salary of at least $17 million for each of the next three years and a club option for 2020, according to Spotrac, before he was officially released Wednesday.
Boston handed out that lucrative contract after the infielder enjoyed a successful seven-year run with the Giants. He tallied 106 home runs and maintained a .346 on-base percentage while winning three World Series titles and earning two All-Star appearances.
His first season with the Red Sox saw him hit .245 with a .292 OBP and 10 homers. That drop in production was compounded by the fact he showed up to 2016 spring training looking out of shape, as the Boston Globe spotlighted:
Although the team initially downplayed the concerns, by the middle of spring training Sandoval found himself battling with Travis Shaw just to keep his spot in the starting lineup.
John Tomase of WEEI passed along comments from Sandoval about the situation at the time.
"You have to earn everything," he said in 2015. "You have to work hard. You have to compete. Things are more interesting when you compete. I don't have a problem competing with every guy here. At the same time, we have a good arrangement, because we're teammates. We're working to be better for each other to make the team better. That's good."
By that point, it had become clear the Red Sox would be best off trying to find any trade avenue possible for the 30-year-old, even if it meant retaining salary to make it work. The efforts became even more urgent after the team announced Shaw won the starting job.
Sandoval went hitless in six at-bats as a backup to start the season before landing on the disabled list with a shoulder injury. Around the same time, former trainer Ethan Banning spoke out about the issues that prevented the infielder from staying in shape, as Evan Drellich of the Boston Herald relayed.
"He's proven to me and shown consistently that he's got to have somebody like me holding his hand doing that. And it's not an exercise thing, it's an eating thing," he said. "Obviously exercise is an important factor in it, a very important factor, but eating is going to be the component that needs to be managed and monitored. We had a chef on staff that cooked all his meals."
He proceeded to undergo shoulder surgery in May and missed the rest of the 2016 campaign.
Perhaps returning to San Francisco will be the spark he needs, as it had quickly become clear he wasn't a good fit in Boston. Without a DH spot, he'll need to be in shape defensively at third, especially since Brandon Belt is locked in at first.
The bigger concern is his performance at the plate. If Kung Fu Panda bounces back, he'll be a welcome addition alongside Belt, Buster Posey, Hunter Pence and Brandon Crawford in the middle of the lineup.