Sizemore will report to the Phillies' Triple-A affiliate, the Lehigh Valley Iron Pigs, on Thursday.
The Red Sox are responsible for Sizemore's $1.25 million big-league salary, which includes $250,000 previously earned in incentives. But if he makes it to Philadelphia, the Phillies would then pay him a prorated portion of the major-league minimum salary. Sizemore had $5 million in incentives built into his contract in Boston, but those incentives aren't part of his deal in Philadelphia, sources said.
Sizemore has an opt-out clause if he isn't called up by the Phillies before the All-Star break.
The 31-year-old outfielder hit .216/.288/.324 in 52 games and 204 plate appearances with the Red Sox this season. He made the club with an eye-opening spring training, homered on Opening Day and got off to an impressive start, with five multi-hit games and a .343/.395/.571 slash line in his first 10 games.
But he tailed off badly afterward, hitting .187/.263/.267 in his final 42 games. He was just 4-for-30, with one extra-base hit and eight strikeouts, in June, when the Red Sox released him.
However, the Phillies, a team desperate for outfield help, are willing to take into account that Sizemore was trying to make a comeback after missing two full seasons, and much of the previous two seasons as well, with knee issues, general manager Ruben Amaro Jr. said.
"More than anything else, when you take that much time off, I think it takes a long time -- much longer than people think -- to kind of get your feet back on the ground, so to speak," Amaro said. "I know Boston gave him the opportunity to play, and he had some level of success, and then he did struggle at times. But we'll see what he's got in the tank. We believe in the athlete. We'll see what he's got left."
Amaro said Sizemore will mostly play center field for Lehigh Valley, but could see time in left and right field as well. Amaro said the Phillies' reports indicated that Sizemore hadn't dealt with "any major issues" in Boston, so they remain hopeful he still has something left.
"Does he run as well as he used to? No," Amaro said. "But he's still very athletic. And our scouts kind of liked some of the things that he did. I mean, really, this is kind of a no-risk, high-reward situation, as far as I'm concerned."
Asked what role the Phillies envisioned for Sizemore in Philadelphia, Amaro replied: "We'd like to have more production in the outfield. It's pretty simple."