Stults, who was scheduled to pitch Saturday in Milwaukee, suffered the injury this week while he was throwing a normal bullpen session between starts.
"It was sort of a freakish thing," Padres manager Bud Black said. "He threw a pitch and felt it in the back of his shoulder."
Andrew Cashner will start for the Padres on Saturday and the team hopes that he can throw upward of three innings. Cashner has been the Padres' eighth-inning specialist.
Cashner will become the 11th different starting pitcher for the Padres this season. Newcomer Jason Marquis, who got the start on Thursday against the Giants after taking Stults' spot on the roster, was the 10th.
Stults, who was claimed off waivers from the White Sox on May 17, is 1-2 with a 4.44 ERA in his first four starts with the team. He allowed six runs (three earned) over 6 1/3 innings in his last start against the D-backs on Sunday.
Stults joins four other starting pitchers on the disabled list -- Tim Stauffer, Dustin Moseley, Cory Luebke and Joe Wieland. All told, the Padres have had 17 disabled list moves involving 15 players.