PHOENIX -- The A's were done in by yet another home run on Saturday, this one a fifth-inning Miguel Montero grand slam that put the D-backs ahead for good in an 8-3 Oakland loss at Chase Field.The devastating blast came just one night after Ryan Roberts handed the A's a heartbreaking defeat with his ninth-inning, walk-off homer against Brian Fuentes, and it put to rest the highly anticipated duel between A's righty Jarrod Parker and the All-Star pitcher Oakland traded for him, Trevor Cahill. Cahill, who also netted the A's Ryan Cook and Collin Cowgill in the deal, was in top form, allowing his former teammates just two runs and seven hits while striking out a season-high seven in 7 1/3 innings to help his new club clinch the series win. "That's the Trevor, the good Trevor we know," A's catcher Kurt Suzuki said. "I was probably more nervous than any other start this year, going against the team I played for for six years," Cahill said. Meanwhile, it was Parker who was responsible for the grand slam -- and an RBI double to Justin Upton three batters prior that the pitcher and his manager believe should have never been. Parker threw a 2-2 changeup to Upton that was perhaps unfairly deemed a ball, and the D-backs outfielder took advantage of his extended time at the plate by driving the next changeup to center field.