A-rod has his hip checked out during the season when he had the issues with the hip. However they thought that the damage was on the same side(right side that he had surgery on back in 2009). They checked out the hip and didn't see any issues. However it was later realized that it was his left hip that was the issue, not his right hip. He felt pain in his right hip, not his left hip.
The reason the surgery is not happening until January is because it is a complicated surgery that requires him too work on strengthening the hip before surgery. This is coming from the medical profession, the doctors. So they clearly are doing this for a reason.