Tiger also had a fusion of vertebrae among other things. A microdiscectomy is simply a minimally invasive procedure to remove part of a herniated disc. He will be out of action for a few weeks at most. The operation usually relieves the pressure on nerves causing chronic pain or spasms. The surgery itself is nothing to worry about. However, the injury that required the surgery is going to send up some red flags around the league. My guess is he had the surgery now because someone took an x-ray and saw the herniated disc. Putting off the surgery would probably hurt him more than getting it over with now. If he put it off until after the draft he would probably miss offseason practices.
But this is not cut and dried. He may never have another problem again and play 12 years in the league.
Update: I saw the tweet claiming he is out for 4 months. That is a long time for a minor surgery. This sounds like more than they care claiming.