Oakland has committed seven errors in five games since the break, including six in its last two contests, and Grant Green, optioned following Tuesday's loss, didn't look sharp in his five starts at second base. He was charged with three errors in that span.
Green, the A's top pick in the 2009 Draft, was also hitless in 15 at-bats to start his big league career.
"Any time a guy like Grant Green, one of our top prospects, gets to the big leagues and gets a taste of it, it's a good thing, regardless of the results," Melvin said. "I told him that last night. His at-bats were good in the last game. For a younger player to come up that's used to playing every day and have to acclimate to not playing every day can be difficult, but you have to learn to acclimate to it if that's your role, and he'll be better for it the next time he's here."
Rosales was hitting .200 with four home runs and eight RBIs in 48 games with the A's when he was designated. His role -- playing shortstop, with Jed Lowrie at second base, against left-handers -- figures to be the same.
"He's a plus defender at every position," Melvin said. "We have been having some defensive issues here recently, and he's one of the guys that can clean that up."
To make room on the 40-man roster for Rosales, the A's designated infielder Vinnie Catricala, who was playing at the Double-A level, for assignment.
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.