Arizona purchased the contract of righty reliever Chaz Roe from Triple-A Reno on Monday.
Cahill was struck by a line drive in his start on June 19 and was hit hard in his next two outings. Manager Kirk Gibson says Cahill has shifted his arm angle and thinks it's because he's been compensating for the hip.
But Cahill has struggled for about six weeks. He's 0-6 with a 7.91 ERA in eight starts since a win May 17 at Miami and is 3-10 overall with a 4.66 ERA.
A first round pick by Colorado in 2005, Roe is making his big league debut. He signed with Arizona after playing for the independent Laredo Lemurs last year.