Oakland, CA (SportsNetwork.com) - In a busy offseason that featured a total roster makeover, the Oakland Athletics will now be without a returning All- Star on Opening Day.

On Friday, the club announced that closer Sean Doolittle is expected to miss the start of the season due to a slight rotator cuff tear.

The lefty was evaluated for shoulder soreness after beginning his offseason throwing program, and an MRI revealed the tear. Doctors believe surgery is not necessary, and he received a platelet-rich plasma injection to decrease inflammation.

There is no timetable for his return.

The 28-year-old Doolittle enjoyed a career season in 2014, recording 22 saves and striking out 89 batters in 62 2/3 innings after assuming the closer's role in May.

In three seasons in the A's bullpen after beginning his professional career as a first baseman, Doolittle owns a 2.97 ERA in 179 innings with 209 strikeouts.