De Vries sustained the injury on a comebacker in his Sept. 8 start against Cleveland. He finished the game, pitching six shutout innings while allowing three hits, but an MRI on Thursday revealed the full extent of the injury.
"It's the reason he was sore," Ryan said. "So we did the right thing in getting that looked at."
Despite the abrupt end to De Vries' surprisingly successful rookie campaign, Ryan said the pitcher has earned a shot at making the Twins' rotation in 2013. De Vries went 5-5 with a 3.99 ERA in 16 starts.
"I think that's fair," Ryan said. "He had a nice year. I'm not sure anybody expected him to come up here -- he's come up here and competed. For the most part he's given us a chance in almost all of his starts. He threw it over the plate. He can show that he's not afraid, which is a good thing. He's athletic enough. He deserves consideration for next year in the rotation. He's pitched himself into that [opportunity].
"Nothing will be given, but he did a nice job for us this year. It's too bad it ended up like this."
Minnesota will transition back to a five-man rotation for the remainder of this season, using Scott Diamond, P.J. Walters, Liam Hendriks, Esmerling Vasquez and Samuel Deduno.