"I think we're going into it with Bruce having one of the five spots," Yost said on Friday during Royals FanFest.
Chen agreed to return to the Royals on Thursday under a one-year deal with a mutual option for 2015. He passed his physical on Friday, but he has yet to be placed on the 40-man roster. That's expected to happen on Saturday, when one player will have to be dropped.
But never mind the paper work, Chen is in Yost's rotation.
"My mindset is you can't evaluate Bruce in Spring Training," Yost said. "He's not one of those guys who can compete, because he's not a Spring Training pitcher.
"But once the bell rings, he's been phenomenal for us. He's been as consistent as anybody can be. You know what you get with Bruce. You're going to go out and get six or seven, for the most part, solid innings that are going to keep you in the game."
Chen, 36, lost a Spring Training battle for the fifth rotation spot last year to Luis Mendoza. But at midseason, Chen came out of the bullpen to replace Mendoza in the rotation, where he remained for the rest of the season. As a reliever last year, Chen was 3-0 with a 2.41 ERA in 19 games. He went 6-4 with a 3.61 ERA in his 15 starts.
Yost's revelation may have been a surprise to reporters on Friday, but not to Chen.
"That's why we signed here, because I was going to start," Chen said.
Chen said his goal was to work hard to be ready by the end of Spring Training and go into the season feeling fresh.
"So in July, August, September and October, I'm still ready," he said.
Yost sounded as if his mind was firm on the matter.
"So unless something just dramatically happens between now and then," Yost said, "Bruce will probably be slotted in one of those five spots."
That leaves left-hander Danny Duffy and right-handers Yordano Ventura, Wade Davis, Luke Hochevar and possibly non-roster invitees Kyle Zimmer and Brad Penny vying for a rotation that already includes two lefties in Chen and Vargas.
"You try to go into it and you think about all scenarios and all cases, and if Danny Duffy doesn't make the rotation to begin with, what more does that do to the 'pen?" Yost said. "It gives us another left-handed arm to get through the All-Star break and then, bam, like Bruce did last year, here comes a fresh, healthy Danny Duffy if somebody's slacking up a little bit."
Chen did not pitch in 2008 because of Tommy John surgery, and he worked his way to Kansas City through Triple-A Omaha in '09. From 2010-12, he was in the starting rotation and led the staff in victories with 12, 12 and 11, respectively.
"I know a lot of things can happen, and nothing is given," Chen said. "And I know we want to win and I'm going to work hard and be ready for the season, but I'm not taking anything for granted.
"I've been there five years, and I'm glad people recognize things I've done in the past. That's why I'm glad to be back here to help the team in any way I can."