Cashner, who has missed 51 games this season with a strained right lat, indicated on Wednesday that he is still having pain in the tendon under his right armpit.
"I can still pitch, but I'm not able to cut my fastball loose," Cashner said. "That's when I feel the pain."
Cashner made two starts since coming off the disabled list on Sept. 1. He allowed two runs over five innings against Arizona on Sept. 7, then gave up six earned runs in 3 2/3 innings in his last start against the Rockies on Sept. 14.
Cashner's velocity has garnered a lot of interest, going all the way back to Spring Training. At one point this season, his fastball was clocked at 103 mph. On several other occasions, he topped 100 mph.
But his fastball never approached that figure after he returned from the disabled list. As it turns out, there was a good reason for that.
"He worked awfully hard, but he still feels it on some pitches. We felt the right thing to do was scale him way back," Padres manager Bud Black said.
Cashner was scheduled to pitch on Friday against the Giants at AT&T Park. Rookie Casey Kelly will now make that start. The Padres, who had been employing a six-man rotation, will now likely scale it back to five for the duration of the season.
As for Cashner, he admitted that the tendon has been bothering him for a while. He's been advised to keep playing catch to break up what he's been told is scar tissue in that area.
Cashner began the season as the Padres' eighth-inning specialist, going 3-3 with a 3.81 ERA in 27 games. He was later sent to the Minor Leagues to get stretched out as a starting pitcher. Cashner made three starts before feeling pain under his right arm. He then landed on the disabled list.
While discouraged that he won't make what likely would have been two more starts, Cashner said he's trying to look at the big picture.
"The biggest thing is getting healthy for next year," he said.