A's assistant GM David Forst said Doolittle will miss the start of the regular season, and there is currently no timetable for his return.
The 28-year-old first felt soreness in his shoulder in December during his offseason throwing program. He was eventually examined by an orthopedist and underwent an MRI, which revealed the tear.
Doolittle had 22 saves, 89 strikeouts and a 2.73 ERA in 62.2 innings last season.
Forst said Doolittle received a platelet-rich plasma injection last week in an effort to relieve the inflammation in his shoulder. Rehab for the pitcher might not begin until the swelling is gone.
- Molly Geary