The moves were made before the Padres played the Colorado Rockies on Thursday.
Maybin was hitting .091 for the Padres before going on the disabled list on April 17 with a right wrist injury. He batted .323 in nine rehabilitation games at Triple-A Tucson.
Alonso was placed on the DL with a bruised right hand, a move that was retroactive to June 1. Alonso has six homers and a team-leading 29 RBIs this season. He hasn't made an appearance in a game since May 31.