To make room on the roster, left-hander Tommy Layne was optioned to Triple-A Tucson.
Richard is 0-4 with an 8.54 ERA in six starts for the Padres.
He had been on the DL with an intestinal virus since May 5th. He made two rehab starts with Triple-A Tucson, going 0-1 with a 2.25 ERA with 12 strikeouts and no walks.
Layne was recalled Friday and appeared in Sunday's loss at Arizona.