Ryu, 35, was pulled after just four innings in Saturday's 7-4 loss to the Oakland Athletics. He lasted less than that in his first start of the season April 10, when he allowed six earned runs over 3⅓ innings in a 12-6 loss to Texas.
"Of course I'm concerned," Toronto manager Charlie Montoyo said Saturday. "We're going to see how he feels."
Ryu, who won 14 games for the Blue Jays last season, is in the third year of a $80-million, four-year contract with Toronto, and is a major piece to the Blue Jays' postseason hopes. Before the injury news became official Sunday, Montoyo expressed concern regarding Ryu's overall body of work this month.
"I'm not sitting here to tell you about track record anymore. He's just struggled these last outings," the manager said. "We're just hoping he makes an adjustment and finds his command."
Left-handed pitcher Ryan Borucki, who was out with a right hamstring injury, was reinstated from the 10-day injured list in time for Sunday's series finale with Oakland.
The Associated Press contributed to this report.