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He confirmed that it involved an ACL reconstruction (via Jay Glazer):
"Robert Griffin III had successful knee surgery early this morning. He had a direct repair of his LCL and a re-do of his previous ACL reconstruction. We expect a full recovery and it is everybody's hope and belief that due to Robert's high motivation, he will be ready for the 2013 season."
Other doctors are questioning whether the optimistic 6-8 timetable is realistic, but it looks like Andrews and RGIII are both targeting a return by Week One of the 2013-14 season.