The MRI showed no structural damage, Cahill said.
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"It was similar to what was going on before," Cahill said. "I was pretty optimistic about the scan. I didn't think it was going to show really much. I don't think it really showed that much change, just back to what I was experiencing before."
Cahill felt the same discomfort after pitching against the White Sox in mid-May when he was with the Padres. Cahill said he was shut down from throwing back then for 10 days, and he wasn't back up with the Padres again for two months.
But Cahill said he doesn't know what his rehab program will be this time around.
"But knowing right now, that I've done this before, I know what helps it and hurts it," Cahill said. "It's not like the first time. It should help me get back sooner."
Cahill said he has felt the impingement for some time.
"I knew it was nothing structural so I was just trying to get through it and deal with it," he said. "But obviously, it just wasn't working out."
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.