The Phillies placed Walker on the 15-day injured list Sunday due to right index finger inflammation.
Walker's move to the IL comes after he labored through four innings in a loss to the Diamondbacks on Friday, giving up four runs on five hits and three walks over four innings. Across 10 starts this season, the 31-year-old right-hander is 3-3 with a 5.60 ERA, 1.49 WHIP and 43:21 K:BB across 53 innings. The Phillies brought up Michael Mercado from Triple-A Lehigh Valley to take over Walker's spot on the 26-man active roster, but Mercado is expected to work as a long man out of the bullpen while Spencer Turnbull moves back into the rotation in Walker's stead.