Montgomery was optioned to Triple-A Scranton/Wilkes-Barre on Saturday.
With CC Sabathia coming off the DL, the Yankees needed to clear a spot on the roster. This has been a successful rookie season for the 24-year-old southpaw. He has a 4.00 ERA, 1.22 WHIP and 118 strikeouts in 126 innings and looks like a solid back-end starter long term. He will be back up when rosters expand in September, but will likely be used in a long-relief role.