"A guy we were impressed with in Spring Training," A's manager Bob Melvin said. "Every time you have an injury or something like this, there's an opportunity for somebody else."
Valdez, who signed a Minor League deal with Oakland in November, pitched in five games for the A's this spring, allowing two earned runs with nine strikeouts in nine innings. He also participated in the World Baseball Classic, appearing in one game for the Dominican Republic.
The right-hander has made two starts for Triple-A Nashville this season, posting a 2.70 ERA after yielding three earned runs across 10 innings. He has 12 strikeouts and hasn't walked a batter.
"I think there's some real deception there in his delivery," Melvin said. "He's got timing issues you've got to deal with, a little bit slower on one pitch and almost like a [Johnny] Cueto type thing, where there's some deception and actually trying to get a bead on him.
"He's pitched well in his short time in Nashville too, so we're excited to get him here."
Valdez's time with the A's could be short, with Graveman (shoulder) expected to miss just one start courtesy of the new 10-day DL.
The A's will have to clear space on the 40-man roster to make room for the Valdez on Thursday.
Jane Lee has covered the A's for MLB.com since 2010. This story was not subject to the approval of Major League Baseball or its clubs.