DeRosa said Friday an MRI confirmed the partial tear. The injury is not in the same tendon that he had surgery on for the second time last season and kept him out most of the year.
But a third tear in his troublesome left wrist could threaten the 36-year-old's career.
DeRosa was injured in the fourth inning of the Giants' 8-5 win on Wednesday night at Dodger Stadium, when he checked his swing on a pitch in the dirt from Clayton Kershaw and immediately grimaced. The Giants recalled infielder Emmanuel Burris from Triple-A Fresno on Thursday after placing DeRosa on the 15-day disabled list.