Chen had been scheduled to face the Blue Jays on Wednesday night. Instead, everyone else will move back one slot -- Yordano Ventura will start on Wednesday and Jeremy Guthrie on Thursday against the Blue Jays, with James Shields starting Friday against the Tigers.
The purpose was to get Yost's front four starters back on four days of rest between assignments, something that Monday's open date disrupted. That's been a juggling act in April anyway because of four open dates and a rainout at Detroit, which caused Ventura to be skipped over for his first start.
Yost said he was leaning toward skipping Chen anyway, but the decision was made easier when the pitcher reported some pain in his lower back in the days after his start last Thursday at Cleveland.
"So, we flew him back [from Baltimore] early Sunday, he had an MRI and they gave him an epidural, a shot to get it to calm down," Yost said.
Chen earlier had been troubled by a sore glute muscle. The Royals believe he might have aggravated his back during a play while backing up home plate at Cleveland. Yost said Chen reported pain running down his leg.
"It's a disc pushing against a nerve, the epidural goes in and shrinks the disc so it doesn't compress the nerve. It's not a major ordeal," Yost said after consulting with the training staff.
"He was willing to battle through it, but I was probably going to skip him anyway this time just to keep everybody on line. It was something he could have continued to work through, but we took the opportunity to go ahead and inject it, and it needs 72 hours, three days, for it to kick in."
Chen's next start was moved back to Saturday against the Tigers, but that will hinge on how well his pain subsides. A decision on his start will be made Thursday or Friday.
"If this is going to do what we think it's going to do, he's going to throw a side [session] on Thursday and pitch his game Saturday," Yost said.
If Chen can't go, Yost wouldn't say who might replace him. However, left-hander Danny Duffy, now in the bullpen, likely would be a possibility.
Yost likes to keep his first four starters on regular rest and if need be, he'll pass over his fifth starter, in this case Chen.
"Especially kind of early in the season," Yost said. "You want to get them sharp, you want to get them going. Later, July and August, it doesn't hurt a time or two. All those years we were in Atlanta, they'd bump the fifth starter all the time to keep everybody else on a five."
That means starting every fifth day with four days of rest.
Shields, Chen and Vargas each has started once with one extra day of rest, while Ventura and Guthrie have both started once with two extra days.
Chen's next start was moved back to Saturday against the Tigers. That, however, still leaves Chen and Vargas, two similar-style left-handers, making back-to-back starts against Detroit. Vargas, the starter for Tuesday night's series opener against the Blue Jays, would be in line to start again on Sunday.
When Yost laid out his rotation when the season began, he took care to separate Vargas and Chen.
"We'd like to get them separated," he said. "We can't do it now, though."