Ryu was shut down for two weeks after feeling discomfort in his shoulder while playing catch on March 22. An MRI revealed no structural damage and showed his shoulder in the same condition as his MRI before he signed with the Dodgers in December 2012.
The Dodgers will be able to reevaluate Ryu within a week to 10 days.
"I don't know how we can judge anything yet. He hasn't thrown a ball," Mattingly said. "Until he gets into that process of throwing and things like that, we'll know a lot more."
Ryu was sidelined with inflammation in his left shoulder twice in 2014, and in both occasions he was able to start exactly 24 days after his last start, with no minor league rehab start in between.
He last pitched on March 17, allowing three runs in three innings against the Rangers in Surprise.
Ryu will open the regular season on the disabled list, and has been ruled out for the Dodgers on April 14, the first date they would need a fifth starter. The Dodgers don't need a fifth starter a second time until April 25, the club's 17th game of the year, but last week Mattingly was skeptical Ryu would even be ready by then.
"We're in a two-week period where he hasn't been able to throw," Mattingly said last week. "It's going to be back to playing catch, more catch, long toss, bullpen, how does he recover."