Third baseman Garin Cecchini -- Boston's No. 4 prospect -- was recalled from Triple-A Pawtucket.
It's likely the Red Sox would have had to part with Sizemore at some point over the next week anyway, as outfielder Shane Victorino will soon return from the disabled list.
"Oh, probably the last couple of days," said manager John Farrell, when asked when the club started to ponder parting ways with Sizemore. "Knowing we've got a lot of moving parts that are going to take place this week and out of respect to Grady, we wanted to make the decision sooner rather than later, if there's other opportunities for him. This has kind of evolved over the last couple of days."
Cecchini, who made his Major League debut in a one-day stint with the club June 1, could be in for another short-term stay as a reserve.
In the coming days, right-hander Brandon Workman will have a ruling on his appeal of a six-game suspension. If the Red Sox lose Workman for his start on Friday, they will call up another starter from the Minors and probably use Cecchini's roster spot.
Sizemore, who returned to the Majors this year after losing two years to injuries, batted .216 with a .288 on-base percentage in 52 games with the Red Sox.
The former third-round draftee was a three-time All-Star and a two-time Gold Glove Award winner during his tenure with the Indians.
Farrell was Sizemore's farm director with the Indians, and delivering Tuesday's news was not easy.
Sizemore was no longer at Fenway Park when the clubhouse was opened to the media.
"Hard, as you would expect," said Farrell, asking how Sizemore took the news. "But he's respectful. Grady Sizemore is about all the good things in this game, from his performance, to the person he is, to his work ethic, the way he plays the game. He's an exceptional person and this is a tough decision to part ways with a guy of his caliber. And though he's trying to do something that I don't know has been done in the game before, we commended him on his work ethic in trying to get back to his previous level."
As it turns out, the clear highlight for Sizemore in a Red Sox uniform came on Opening Day, when he marked his comeback with a home run at Camden Yards.
Quite simply, Sizemore wasn't able to perform consistently, and the fact that several other Boston outfielders have struggled at the plate this season didn't help his cause.
Daniel Nava has regained his stroke of late and Brock Holt has turned himself into an invaluable super-utility player, leaving Sizemore without a role.
After Jacoby Ellsbury departed as a free agent, the Red Sox decided to take a low-risk gamble on Sizemore, agreeing on a guaranteed $750,000 deal. With performance bonuses, Sizemore wound up earning $1.25 million during his time in Boston.
Red Sox general manager Ben Cherington wouldn't be surprised if Sizemore eventually regains his groove with another team that can give him more time.
"As we explained to him, it was a really tough decision because of the person that he is and how much we think of him," said Cherington. "We were certainly hoping and thinking that this would and could have turned out differently, and we were still hoping it would up until recently. It just got to the point where we didn't think we could give it more time. We're trying to find ways to create a better roster alignment for John and his staff, and so we made the decision to make the move."