Velasquez threw only two pitches in Wednesday's home outing against the Chicago Cubs.
"The MRI was pretty clean," Phillies manager Pete Mackanin said. "We'll see what the medical people have in store for him."
In 12 starts, Velasquez is 5-2 with a 3.65 ERA.
"I guess we'll shoot for 15 days and go from there, but I don't know," Velasquez said. "It's just a strain and we're going to treat it the way it should be treated. I guess it's just downtime for right now."
Philadelphia recalled RHP Luis Garcia from Triple-A Lehigh Valley.
The Phillies open a three-game series Friday against the Nationals.