Lynn has faced some relative struggles of late and was pulled after just two-plus innings in the Cardinals' 8-5 win over the Reds. He gave up four runs on six hits before being replaced by Kelly, who shined in his three innings of work, giving up one run on four hits while striking out six.
Lynn hasn't earned a win in a month but holds a 13-5 record with a 3.93 ERA, and manager Mike Matheny assured that this won't be the end of the road for the 25-year-old righty.
"We want Lance to stay sharp," said Matheny, who added that the move is only confirmed for one start. "We never know how this stuff works out. We'll give him a little bit of time here to get things right. He could be one of the most important pieces out there every fifth day and could be one of the most important pieces of the bullpen, too."
Lynn started the season 8-1 with a 2.54 ERA but is 0-1 with a 6.56 ERA in five August starts.
"These are some of the greatest learning experiences for a young pitcher," Matheny said. "He really just dominated the first half of this season with very few trials. Those trials and those struggles and the perseverance you develop through those define you as a pitcher in your career moving forward."
The rotation isn't a new spot for Kelly, who has made 12 starts this season, as recently as Aug. 14.
Kelly, who was thrust into Friday night's game on short notice, is 3-5 with a .375 ERA as a starter this season after making the first start of his career June 10.
"When I first started starting it didn't take much for me to get hot," said Kelly, who has made 11 quality starts this season. "I end up throwing more pitches to get a better routine, though, to become a starter. Whether you're doing bullpen or starting, you want to [have] damage control as much as you can and whenever you can."
Lynn has had early-inning issues haunt him this season and struggled with his command recently but said Saturday he's ready to pitch.
"It's a work in progress and we're still working," Lynn said. "We'll get better."