It's certainly a bad injury for that already thin defensive line. I don't necessarily think its a season killer, though. The defense has been playing at a pretty high level all season and a large part of that has been a much improved secondary and some outstanding play on the second tier by Mayo, Hightower, and even Spikes.
Tommy Kelly had himself a strong game last night and the hope is that he can remain playing at that level. They are not going to find anyone to replace Wilfork but can muster up some stop gaps on the d-line although I have some doubts on Vellano (size & speed).
I think a season ending injury to a guy like Talib or Mayo would've been more devastating for this team right now.
It's awful about Wilfork, though.