Sandoval, 30, slashed .212/269/.354 with four home runs and 12 RBI through his first 32 appearances this season.
The Red Sox signed Sandoval to five-year, $95 million contract in 2015 after he earned two All-Star nods over the course of seven seasons with the San Francisco Giants, but his abbreviated stint in Boston was marred by injuries, subpar production and concerns about his weight.
Jon Morosi of MLB Network pointed out that the $49.5 million left on Sandoval's contract is the second largest amount of dead money on a contract in MLB history, second only to Josh Hamilton.
After Sandoval batted .245 with 10 home runs and 47 RBI in 2015, he was limited to three games in 2016 after he was forced to undergo season-ending shoulder surgery.
And while he made a full recovery in time for the start of the 2017 campaign, a right knee sprain sent him back to the disabled list in late April and forced him to miss all but two games in May.
But even when he was healthy, Sandoval failed to live up to lofty expectations with his bat and his glove.
All told, Sandoval finished his Red Sox career with minus-2.0 wins above replacement after he accounted for a robust 20.8 with the Giants.
According to Morosi, the Red Sox will be on the hook for the remainder of Sandoval's deal, which includes $18.6 million salaries in 2018 and 2019 along with a $5 million buyout on his club option for 2020.