Casilla, who recorded two outs in the eighth inning before exiting with a trainer, suffered a similar injury in Spring Training and believes it to be minor.
"I felt my shoulder [was] a little bit tight when I started getting loose today," Casilla said through team interpreter Juan Dorado. "It was something that was bothering me in Spring Training, but I don't think it's something that will take long to heal or linger."
The 37-year-old veteran has a 3.32 ERA in 21 2/3 innings for the A's this season.
Jane Lee has covered the A's for MLB.com since 2010. Follow her on Twitter @JaneMLB.