"After performing an MRI today on Andrew's right elbow, our doctors have indicated to me that he has elbow soreness and irritation," Padres general manager Josh Byrnes said in a statement. "Based on our doctors' recommendation, we will be proceeding with rest and rehabilitation for his recovery."
Cashner had his MRI in San Diego on Monday, one day after the team returned late Sunday from a six-game road trip. He was scheduled to start Sunday's series finale in Denver against the Rockies, but he told manager Bud Black on Friday of soreness in his elbow.
Cashner was upbeat and optimistic at the time that this setback wasn't anything catastrophic.
"I think it just has some inflammation in it," he said. "Hopefully it's not more than 15 days. I think there was some extra soreness this time, but I'm not worried at all about my ligament. I have been cutting my fastball loose with no pain."
Cashner, who ranks eighth in the National League with a 2.35 ERA, last pitched on May 13 on the road against the Reds, where he allowed one run over seven innings, with one walk and six strikeouts. He threw a side session in Cincinnati on Thursday and had soreness Friday.
It was similar to the soreness Cashner experienced after a start against the Royals on May 7, Byrnes said.
"More than anything, we wanted to err on the side of caution with the soreness he has had off and on," Black said on Friday. "He came in and was quite honest, and we should really consider this, because I want to nip this in the bud instead of trying to push something."
Cashner is 2-5 and opponents are hitting .235 against him this season. According to FanGraphs, Cashner has leaned more on his fastball this season (72.8 percent, up from 63.4 percent in 2013) and slider (18 percent, up from 14.4 percent a year ago).
During that start against the Reds, Cashner allowed three hits and one run in the first inning, but he settled down thereafter. He was still throwing 94-95 mph in his last inning of work.
"It was probably one of the best starts, as far as just my body and the way my arm felt," Cashner said.
Earlier this season, Cashner nearly became the first Padres pitcher to throw a no-hitter, as he allowed one hit in a shutout of the Tigers on April 11. To date, he has only surpassed the 100-pitch mark twice, and he hasn't thrown more than 108 pitches in any single start.
Losing Cashner for any length of time is a blow to a starting rotation that had maintained all five of its starters since Opening Day -- as San Diego was one of only three teams to have done so. The others are Boston and Milwaukee.
Rookie Donn Roach, who had pitched exclusively out of the bullpen, started for Cashner on Sunday at Coors Field and allowed four runs in three innings. The Padres haven't announced if Roach will start again when his turn in the rotation comes around again Saturday against the Reds.