The defense is a mess, 3 or 4 rookies are not going to solve the issue. I would argue that a mid-high end vet would have more of an impact than any draft pick.
Both perspectives (defense = mess, vet short-term impact > rookie) are aligned with conventional wisdom, but I'm inclined to believe it is not that black-n-white.
"Defense is a mess"...The stats back this up but I have a hard time thinking this is fundamental. Outside of the Steeler game, the defense has either started well enough to allow the team to grab a good lead or stiffened up to allow the team to come back from an early deficit. If the defense was simply incompetent, good offenses (the Pats have played 5 top 10 passing offenses) would be able to consistently score across an entire game. That just hasn't happened.
Have to bear in mind that the Pats are missing 6 starters or top role players on defense (Carter, Pryor, Wright, Bodden, Dowling, Barrett) and played a number of games missing 2 others (Chung, Spikes). Obviously this defensive squad that you call a "mess" wasn't the plan in late August. So taking this improvised "mess" and projecting it as the "opening bid" for next year doesn't seem logical.
Instead of looking at what is missing, look at what is already in place. Assuming both Carter and Anderson are back (they seem to like NE and are unlikely to get big bucks elsewhere), here is what they have:
DBs
===
CB: McCourty, Dowling
S: Chung
BASE 3-4
=======
NT: Wilfork
DE: Deaderick
OLB: Ninkovich
ILB: Mayo, Spikes
BASE 4-3
=======
DT: Wilfork, Love
DE: Carter
OLB: Ninkovich, Mayo
ILB: Spikes
SUB
===
DT: Pryor
DE: Carter, Anderson
LB: Mayo
So what is missing or ripe for upgrade?
DBs: Another CB (Arrington) for Nickle and coverage S (Barrett)
BASE 3-4: DE (Warren, Ellis), OLB (Cunningham)
BASE 4-3: DE (don't want Anderson in base if avoidable)
SUB: Rush DT (have to cut back Wilfork's snaps), coverage LB (Fletcher)
"Vet has more of an impact"...So a coverage S (or CB if McCourty/Dowling transitioned there) is a primary, 3-down need. Not sure you will find much in FA so it will likely take a premium draft pick. The other needs are important roles but there are already system-ready vets on the roster for these roles. Not sure that investing in non-system-ready vets (even more talented ones) is necessarily the best answer.
Since these roles will have limited reps and responsibilities, they are good places to incorporate rookies. If you are primarily eating blockers or rushing the passer or covering TE/RB routes, you can focus on just that during workouts/camps to be effective your rookie year. If a vet opportunity comes up and the price is right, by all means sign them up. But if FA isn't all that, getting 3-4 rookies in the draft to fill these roles can still pay major dividends right away. As always, just need to find the right guys. With 4 top 60 picks and a top-heavy draft, I'm confident this can happen.