Cardinals manager Mike Matheny said Carlos Martinez will go to the bullpen.
"We are just in a spot where we've got to try and figure out what is best for our club," said Matheny, noting that Martinez is a proven commodity in the bullpen.
Kelly lost a competition last spring to Shelby Miller, but moved from the bullpen to the starting rotation later in the year. He started 15 games for the Cardinals in the regular season and four more in the postseason and has a 3.08 ERA.
Martinez thrived in the setup role late last season, including in the playoffs. He will return to eighth-inning duty, setting up closer Trevor Rosenthal, despite allowing only three earned runs and striking out nine in 15 1/3 spring innings.
"I believe that he is eventually going to be a starter and I believe he is going to be a very good one, but right now we need him to help our club out of the bullpen," Matheny said.
Kelly's control fluctuated in Sunday's start, during which he struck out five but also walked four. He allowed two runs and two hits in the sixth and didn't get an out before being lifted.
"I wouldn't say it was great, by any means," Kelly said before learning he would be in the rotation. "The walks are just something that I absolutely don't like."
While Matheny named Kelly to the starting rotation, he declined to give the order in which the starters will appear. Adam Wainwright will be the Cardinals Opening Day starter. He will be followed, in an order to be announced later by Lance Lynn, Michael Wacha, Miller and Kelly.