Herndon, 0-1 with a 4.70 ERA in five games, is having tests on the elbow in Philadelphia on Tuesday.
The Phillies called up right-hander Brian Sanches from Triple A Lehigh Valley to replace Herndon in the bullpen. Manager Charlie Manuel said Sanches, who was to be available for Tuesday night's game against the Braves, "suited our needs" because he can pitch in any role.
Sanches is beginning his second stint with the Phillies. He pitched for Philadelphia from 2006-07 before one season in Washington and three with Florida. The 31-year-old Sanches is 13-6 with a 3.58 ERA and five saves in 189 career games.