Blackburn, who also had a bone chip removed from his right elbow in October, will not be ready in time for Spring Training. Twins pitchers and catchers are set to report on Feb. 12, and Blackburn will start his rehab once the cast is removed.
He was considered a candidate to be the club's fifth starter after the Twins added Vance Worley, Mike Pelfrey and Kevin Correia to a rotation that already includes Scott Diamond. Other rotation candidates include Kyle Gibson, Liam Hendriks and Cole De Vries.
Blackburn, though, struggled last year, posting a 7.39 ERA in 19 starts before being outrighted off the 40-man roster in August.
Blackburn, who is 43-55 with a career 4.85 ERA over 145 games (137 starts) in six years with the Twins, is set to earn $5.5 million this season.