The Athletics signed Lopez on Monday to a minor-league contract that includes an invitation to spring training.
Atlanta released Lopez on Monday.
Atlanta reassigned Lopez to minor-league camp March 5.
Lopez signed a minor-league contract with Atlanta on Saturday.
Lopez elected free agency Tuesday, Charlie Goldsmith of The Cincinnati Enquirer reports.
The Reds returned Lopez to Triple-A Louisville on Sunday.
Lopez was a late addition to Saturday's starting lineup at second base after Noelvi Marte was scratched with a facial contusion. He went 1-for-2 with an RBI and was ejected for arguing a called third strike.
The Reds added Lopez to the active roster as Stuart Fairchild's (illness) replacement Saturday.
Lopez cleared waivers and was sent outright to Triple-A Louisville on Saturday.
Lopez was designated for assignment by the Reds on Thursday.