So, I just went through the PFR 2018 Pats snap totals and noted which guys saw significant time on D last year.
Our "starting 2018 defense" (the 11 players with the most snaps):
EDGE: 98 Flowers, 70.2%
DL: 93 Guy, 49.8%
DL: 90 Brown, 43.7%
LB: 54 Hightower, 74.2%
LB: 53 Van Noy, 90.7%
CB: 24 Gilmore, 97.2%
CB: 30 McCourty 80%
CB: 31 Jones 49.4%
S: 32 McCourty 96.3%
S: 23 Chung 85%
S: 21 Harmon 61%
Guys who missed that cut but still saw a lot of time (or a bigger role at a certain part of the season before or after injury such as Bentley, Simon):
EDGE: 91 Wise 41.4%
LB: 52 Roberts 41.2%
CB: 27 Jackson 37.9%
DL: 70 Butler 36.3%
DL: 71 Shelton 31.1%
EDGE: 94 Clayborn 30.5%
EDGE: 55 Simon 17.7%
LB: 51 Bentley 13.2%
Guys in bold left the team this offseason. I feel fairly confident with Bennett, Pennel, and Winovich respectively replacing the roles/snaps of Flowers, Brown, and Clayborn. I don't see any of those changes being downgrades, but potentially upgrades. This leaves Shelton as the lone player to be replaced, which was 31.1% of snaps last year (of generally mediocre quality, Super Bowl notwithstanding). I'm not terribly worried about replacing him because he wasn't very good to begin with; he's also still available.
Now for the returning players who might have their workload lightened by new acquisitions. I've made their names italic. With Bentley returning, Van Noy and Hightower could be kept fresher, and Roberts doesn't have to be on the field as much (even though I'm much higher on him than most are). Joejuan Williams should enter the outside CB/TE-covering safety rotation and potentially keep Gilmore, the McCourty bros, and Chung fresh.
Most of this defense is intact, which is definitely a good thing, but I want to remind everyone that what we saw in the Super Bowl is not what we saw the previous 17 games and is not what we will see in September. We didn't need to simply maintain the roster by replacing the lost players; we needed to upgrade. I think we did it.