"Last couple innings, it was barking pretty good," Cahill said. "Then over the last couple days, I tried to play catch, and it just wasn't rebounding at all. I didn't want to push it. Give it more time, and see if I can rebound."
Right-hander Jarred Cosart will start in place of Cahill on Thursday. Injuries and inconsistencies have prevented Cosart from taking ownership of a spot in the Padres' rotation since he arrived at the Trade Deadline last season.
Padres skipper Andy Green made it clear he views this as a chance for Cosart to make his case as a long-term starter.
"It's time for him to rise up and claim something that you really want as a Major Leaguer," said Green. "You don't get endless supply of opportunities. Sometimes things happen you can't control, whatsoever. But where he is with us: Go run with it. We want to see him do great. We believe in what he has in his right arm."
Quackenbush will slide into Cosart's place in the bullpen, with a combination of Miguel Diaz and Craig Stammen taking over Cosart's long-relief role.
Quackenbush has certainly worn quite the path between El Paso and San Diego. Since his 2014 debut, he's been optioned 12 times. Already this season he's been back and forth four times.
"You can't really prepare for it; you just get the call and roll with it," said Quackenbush, who has a 3.68 ERA in 7 1/3 innings this season. "In that aspect, I've just kind of gotten used to it. You've just got to be ready, I guess, because it could happen at any time."
It's Cahill's second disabled list stint this season. The first resulted from a minor back strain in April, and he missed only one start before returning after the minimum 10 days.
With Monday's off-day, Cahill could miss only one more start, should he return after the minimum 10 days. But it's worth noting the shoulder injury was sustained while pitching. When Cahill tweaked his back, it came during a workout, and he almost avoided the DL entirely.
"A shoulder can be a little bit more serious than the back thing I was going through," he said. "But I trust in the training staff. ... It's just one of those things. Hopefully it's a minor setback and I can still make 29-30 starts."