The struggling right-hander agreed to go to the minors Monday, a day after he pitched poorly in relief. A decision on where to send Eaton has not been made.
The Phillies will fill his spot before Tuesday's game at Washington. It's likely left-hander J.A. Happ will be recalled.
Eaton was 4-8 with a 5.80 ERA in 21 games this season. He pitched out of the bullpen twice after being removed from the starting rotation. Eaton allowed three runs in two innings in a 12-10 victory over Atlanta on Sunday.
Since signing a $24.5 million, three-year deal as a free agent before last year, Eaton has been a bust. He was 10-10 with a 6.29 ERA in 30 starts last season, and was dropped from Philadelphia's postseason roster.
It's the second time the Phillies have sent a veteran pitcher to the minors this season. Brett Myers, the team's Opening Day starter, was demoted on July 1 after going 3-9 with a 5.84 ERA in his first 17 starts.
Myers made four starts in the minors and returned to the Phillies last week. He had a so-so outing against the New York Mets, and is scheduled to face the Nationals in the opener of a three-game series Tuesday.
Eaton lost his spot in the rotation after the Phillies acquired Joe Blanton from the Oakland A's on July 17. The 31-year-old Eaton had the right to refuse the assignment to Triple-A.
Happ had a 3.27 ERA with no decisions in two starts filling in for Myers. He probably would pitch as a reliever this time.
Eaton was originally chosen in the first round by Philadelphia in the 1996 amateur draft. He never won more than 11 games nor pitched 200 innings in any season, but the Phillies gave him a hefty deal after he was 7-4 with a 5.12 ERA in 13 starts in his only season with Texas in 2006.
The defending NL East champion Phillies were one game behind the Mets entering Monday's games.