On paper, this defense absolutely has the ability to be a top 5 unit. Whether they actually play to that level remains to be seen, but the talent and depth is there.
Very few teams have the talent and depth that the Pats have at DT or in the secondary. Given how Chris Jones, Sealver Siliga and Joe Vellano played last year and they are likely at the bottom of the depth chart behind Vince Wilfork, Tommy Kelly, Dominique Easley and Armond Armstead, few teams have that kind of talent or depth at DT. And the Pats have 7 DBs capable of playing at a solid level or better: Darrelle Revis, Brandon Browner, Devin McCourty, Alfonzo Dennard, Logan Ryan, Kyle Arrington (in the slot) and Duron Harmon. Again, few teams have that kind of depth, or that much quality talent. Add in 2 solid DEs in Chandler Jones and Rob Ninkovich and 3 very good LBs in Mayo, Hightower and Collins, and we have at least 12-13 guys capable of playing at a starting level, and another 3-4 who can get quality reps and not be a liability. Very few teams have that kind of depth, or so few holes.
Of course there are questions: Wilfork's health, depth at DE and LB, and the safety position, for starters. If Wilfork can recover to a meaningful level, if Michael Buchanan can develop, if Will Smith can contribute, if the Pats can find depth at LB, and if the safety position gets sorted out, then things will be a lot clearer. But all teams have issues, question marks or holes right now, including some of the other potential top 5 defenses:
- Seattle: lost 5 quality players in Chris Clemons, Red Bryant, Clinton McDonald, Brandon Browner and Walter Thurmond. Depth at DB and DL is much thinner than it has been in the past. Take a look at this breakdown of the defensive roster and see how many holes there are, or areas where they are counting on guys with no prior experience to provide depth:
http://espn.go.com/blog/seattle-seahawks/post/_/id/6194/seattle-defense-position-by-position
- San Francisco: Navorro Bowman is coming off a horrific injury and will probably miss half the season; Aldon Smith could be suspended; the secondary lost Donte Whitner and Carlos Rodgers and has a bunch of new pieces; Justin Smith can't go on forever.
- Carolina: Greg Hardy had a major off field incident and could face discipline, or be a distraction. Almost the entire secondary left in FA.
- Arizona: lost a key player in MLB Karlos Dansby, and LB Daryl Washington could be facing discipline.
- Cincinnati: lost their DC, and a key piece of their dominant DL in FA DE Michael Johnson.
- Tampa Bay: added some nice talent in DE Michael Johnson, DT Clinton McDonald and CB Alterraun Verner but lost CB Darrelle Revis. They have a lot of new parts and a new defensive scheme to adjust to.
- NY Jets: they have no secondary, despite using a 1st round pick on Calvin Pryor.
It's too early to tell how the Pats' D will come together, but they lost no key players on defense other than Aqib Talib, who they upgraded with Darrelle Revis, and they have as much or more depth and talent as just about any team in the NFL.