Ramsey sprained both AC joints last season. It seems like he tried non-surgical methods on both shoulders and it appears that worked for one shoulder, but not the other.
I'm guessing they waited for the other shoulder to respond as long as they could, opting for surgery late enough in the year to give the shoulder every opportunity to respond but early enough in the off season so his status for Week 1 isn't in jeopardy.
For injuries that have no chance to heal on their own (i.e., a torn ligament) they do surgery pretty much as soon as they can. For injuries that have a chance to heal on their own (i.e., sprained AC joint) they try non-surgical approaches first and only do surgery as a last resort.
AlbaNY_Ram
