It's common for people to suffer inflammation flareups months after a serious joint injury. Urlacher obviously felt okay when he was staying off it, but once he started practicing the joint started swelling up on him, causing pain and discomfort. Hence the need for the surgery.
Hopefully they'll monitor the swelling now and he'll be ready to go in 3-4 weeks time. I think we can get by okay for the first handful of games without him, if our offense is even half as good as advertised.