Curtis was optioned to Triple-A Lehigh Valley on Monday, Stephen Gross of the Morning Call reports.
Curtis was battling for a role as a left-handed reliever in the Phillies' bullpen but was always a long shot behind Adam Morgan and Hoby Milner. The 25-year-old has received brief callups in each of the past two seasons, recording a 4.57 ERA (and a 6.71 xFIP) in 21.2 major-league innings. He fared better at Double-A Arkansas in the Mariners' system last year, tossing 51.1 innings with a 3.51 ERA. He'll get his first taste of Triple-A action at the start of this season and could be back in the big leagues in a low-leverage role at some point in 2018.