"I don't think it's serious," Ryan said. "He came out of the game, but I don't think it was serious."
Stewart, 19, has fared well in his first full professional season after being taken with the No. 4 overall pick in the 2013 First-Year Player Draft. The right-hander has posted a 2.59 ERA in 19 starts with 62 strikeouts and 24 walks in 87 innings.
Ryan said he remains hopeful Stewart will be able to return to action this season. "I hope so," Ryan said. "We still have season left. We've done a number of tests on him and haven't been able to find anything wrong."
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.