Due to Mayo's injury, I don't see the 4-3 as a good fit for this defense. To properly run a 4-3, you need some speed and range at LB. Mayo provided this when he played at ROLB. Spikes would man the MLB position, and Hightower found himself at LOLB. New England played a certain alignment of 4-3 that put these guys in ideal positions to succeed. Forgive me, as I don't recall the exact formation alignment. It was either 4-3 odd, 4-3 over, 4-3 under, or 4-3 even.
However, with Mayo, arguably our rangiest LB, gone, I don't see Hightower and Spikes holding up in a 4-3.
We saw a good bit of the 3-4 on Sunday, and I'm surprised this isn't getting more attention from fans here. It looked good when it was used in the earlier parts of the game. It seemed rather clear to be that the 3-4 was their base defense going into the game.
While losing Mayo and Big Vince hurts, I think the following 3-4 could minimize the losses.
Kelly - Forston- Armstead
Ninkovich - Fletcher - Hightower/Spikes - Jones
I think a defensive line of Kelly, Forston, and Armstead could do some good things. Slide Chris Jones and Vellano in as backups to Kelly and Armstead at the 5-tech, DE position. Forston is the weak link, and he's obviously no Vince, but he's a big-body who can take on a few blocks. He doesn't need to be great, just needs to not be bad.
At the LB position, I see Ninkovich and Jones as natural fits at their respective positions. Ninkovich would be playing the Vrabel role, as the swiss-army-knife, do-it-all, weakside OLB in the 3-4. He can rush the passer, play the run, and cover when needed. This position fits his skill-set perfectly. Jones is an adequate "elephant". He's athletic enough and smart enough to drop into coverage on occasion, but he would be primarily rushing the passer, a role that he has thrived it. Fletcher would play the Bruschi role; the more coverage-oriented MLB. Fletcher has shown flashes and he could thrive in this defense. He's fast, instinctive, and great in coverage. While his run defense is suspect, he is adequate enough. Lastly, Hightower and Spikes could rotate through at the "thumper" position next to Fletcher. Think Ted Johnson -- a big, hard-hitting LB, good for an occasional blitz.