Girardi said Phelps first felt pain in his forearm after his start Thursday against the Twins.
The right-hander underwent an MRI on Friday that revealed a minor strain.
The Yanks considered skipping Phelps' next turn in the rotation rather than placing him on the DL. But they ultimately decided to put him on the DL to make sure that he's healthy for the second half of the season.
CASHMAN TALKS CATCHERS: Brian Cashman said Friday that he's open to looking outside the organization for a catcher, but there isn't much to choose from.
Francisco Cervelli has a stress reaction in his right forearm and is most likely out until at least Aug. 1, so Cashman was asked if he's looking at any catchers to replace Cervelli.
"I've been open for anything that makes sense," he said. "But the catching market is completely barren."
Cashman said he was more likely to promote Triple-A catcher J.R. Murphy than look outside the organization for a backstop.
ALSO:Derek Jeter begins his rehab assignment with Triple-A Scranton/Wilkes-Barre on Saturday afternoon; Alex Rodriguez will play in Brevard County on Saturday. ... All-Star selections will be announced Saturday night.