"Bruce has got some stiffness in his back," Yost said. "We've been on a six-man rotation the last two or three times through because of the off-days. We can get Bruce's back to calm down a little bit and move Shields up to a regular five and move Vargas up to a regular five and slide Bruce in after Vargas.
"He could have pitched [Thursday]," Yost said of Chen. "But we're going to give him a couple of extra days to get it out of there."
Chen said the injury was actually a tightness in his glute.
"I could have pitched [Thursday]," Chen said. "But we had an off -day [Monday]. We have a chance to put a couple of guys in their five-day [routine] and give me a couple of extra days.
"Right now, it's early in the season, everyone can do their thing and I can get this resolved. I've had it for over a week now. The preparation in between [starts] wasn't what I wanted it to be."
Chen lasted only 3 2/3 innings in Friday's loss at Minnesota, allowing seven hits and six earned runs.
He is happy to be getting another shot at the Twins. "I'm glad I can go out there and perform against a team in our division, especially one that swept us before," he said. He will miss a chance to pitch against Houston. He has a career 3-1 record and 2.72 ERA vs. the Astros.
Shields said he was told Tuesday that he had been moved up to Thursday in the rotation.
"I'm ready to go," he said, adding it didn't matter to him when he pitched. "You have to make adjustments in this game and roll with the punches."
Gene Duffey is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.