Pomeranz was 2-3 with a 4.40 ERA in eight starts with the A's, his last outing coming Monday in Houston, where he exited with shoulder tightness after just 4 1/3 innings. The left-hander flew back to Oakland on Wednesday for more tests.
Leon joins the A's for a second time this season. He was originally promoted April 22, and he made his big league debut that day in Anaheim before being optioned to Nashville. Since then, he's posted a 2.65 ERA in three starts for the Sounds.
Look for the right-hander to provide length out of Oakland's bullpen.
"As like most things right now, we're in a little bit of a flux," said Melvin. "Probably more length, middle innings type of thing, depending on availability of guys on certain days."
Jane Lee is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.