I don’t think they’ll go back to the 3-4, at least in a traditional sense when it was a straight up nose tackle and two 5-techniques. They just don’t have the two-gapping defensive ends. Deaderick is almost a fit, but he’s certainly nowhere close to what Richard Seymour was. 

Once they traded Seymour in 2009 and then lost Ty Warren in 2010 they just couldn’t run it effectively. They tried to plug Jarvis Green in for Seymour in 2009 but was rough against the run.

Mayo is certainly a better fit in the 4-3, where he can play more in space. They were playing him at the weak inside linebacker position in the 3-4 and he tended to get swallowed up down in tight.

Here’s a couple articles that expands on this a little more:

Breaking Down Jerod Mayo’s All-Pro Caliber Season in Patriots Defense

 Why the Patriots Must Alter Defensive Scheme Yet Again in 2013

Not gonna lie, it will be a little scary, Welker was Brady’s third down blankee. But between Gronk and Hernandez he should be okay. The offense would have to adjust, but they’re in far better position to do it now with their stable of tight ends and running backs. The more I think about it the harder it is for me to justify breaking the bank on Welker when there’s ZERO wide receiver and cornerback depth on the roster, and significant needs at defensive tackle. I’m not sure how it will go, but the Patriots were a top-10 offense before Welker and they’d most likely still be one after him.