Chen was supposed to throw a side session Thursday to test his readiness to pitch after missing a scheduled start Wednesday night because of lower back problems.
"He just didn't feel good," manager Ned Yost said. "His back was still stiff and he still felt some pain, even when he was warming up. He didn't even get to the mound."
Left-hander Danny Duffy, despite big problems in his past two relief outings, will substitute for Chen on Saturday.
Duffy was sent to Omaha at the end of Spring Training to get stretched out as a starting pitcher in case the Kansas City rotation needed help. However, after one start, he was recalled from Omaha to shore up KC's bullpen.
His first four outings went well as he pitched 8 1/3 scoreless innings with five hits and two walks given up and 11 strikeouts. The next two appearances, however, were eyesores.
Saturday night in the 10th inning at Baltimore, he faced three batters. Duffy hit the first one, then fielded two successive bunts and made throwing errors on both. That ended his outing and Louis Coleman gave up the game-ending hit in a 3-2 Orioles win. Wednesday night against the Blue Jays, Duffy took over in the sixth and immediately hit one batter and walked the next. He was pulled immediately but both runners scored for a 2-2 tie. The Royals went on to win, 4-2.
"I believe he'll be fine," Yost said. "I'll have some coverage if I need it."
Yost estimated he could throw 65 to 70 pitches, so another pitcher would have to be designated to "piggyback" with him.
"We get through Friday and we'll have a better idea of formulating a game plan on who's available and who's not available," Yost said.
Chen had been bothered by what was described as a gluteal muscle strain for about four weeks. Earlier this week it was determined that he had a disc issue as well and he was given an epidural injection.
"I talked to Nick [Kenney, head trainer] about it and he thinks it's going to progress quickly and it shouldn't be much past the 15-day ordeal, but you've got to go day-to-day with it," Yost said.
"He's got a disc that's bulging against one of his nerves and these injections shrink the disc and calm the inflammation down. It's just waiting for that to happen."
Yost speculated that Chen would be ready to pitch before his time on the DL expires.