At this point, we know that Mayo, Guyton, Banta-Cain, McKenzie, and Ninkovich will be part of the mix at linebacker for next season. Beyond that, there's a lot of doubt.
On the outside, you have one starter in Banta-Cain, who can rush the passer at a high level and has improved against the run. The Pats still need another guy on the outside, a bigger body like Vrabel who can defend against the run and also provide some pass rush. Thomas theoretically would fit that mould perfectly, which is why he's here in the first place, but he will not be part of the picture next season. Look for the Pats to address this need EARLY in the draft. Ninkovich is only a special teamer and reserve. Crable can't be counted on as anything more than a 4th OLB; you still need guys to replace Burgess, Thomas, and Woods (if he leaves) as part of the rotation. Drafting a Woodley-type would be huge. (Graham?)
On the inside, you have a good mix of young players in Mayo, Guyton, and McKenzie. However, I think the Pats would still like to rotate one more guy in there. Larry Foote will be a free agent and seems like a possibility; he has experience playing in Pittsburgh's 3-4.