Johnson was optioned to Triple-A El Paso following his acquisition Saturday from the White Sox, Dennis Lin of the San Diego Union-Tribune reports.
Though the Padres have a vacancy in their rotation with James Shields heading to the White Sox, manager Andy Green will go with long reliever Luis Perdomo in Shields' place Sunday. However, Perdomo is sporting an ERA over 10 on the season, and probably profiles better as a bullpen arm at this point, so it might not be long before Johnson gets a chance to claim the No. 5 spot. According to MLB.com's AJ Cassavell, Green has already said there's a "pretty good possibility" that Johnson takes Shields' spot in the rotation long-term, so if the 26-year-old quickly thrives at El Paso, a promotion could be imminent.