The Bears' defense has been good so far. It has also been very concentrated, with the starters playing a ton of snaps. Here are the snap rates, from
PFR:
100% LB T.J. Edwards
100% LB Tremaine Edmunds
100% CB Jaylon Johnson
100% SS Jaquan Brisker
100% FS Kevin Byard
99% CB Tyrique Stevenson
87% CB Kyler Gordon
84% DE DeMarcus Walker
72% DT Gervon Dexter
71% DE Montez Sweat
67% DT Andrew Billings
44% DE Darrell Taylor
27% DT Chris Williams
20% DE Austin Booker
13% LB Jack Sanborn
10% DT Byron Cowart
3% DT Zacch Pickens
0.4% DE Daniel Hardy
0.4% CB Jaylon Jones
0.4% CB Josh Blackwell
0.4% CB Terell Smith
The top 11 players are averaging a 89% snap rate. On average there are:
1.19 players on the field who are not among the top 11
0.74 players on the field who are not among the top 12
0.47 players on the field who are not among the top 13
0.28 players on the field who are not among the top 14
The back 7 is extremely stable, with the same 7 guys on the field about 87% of the time. 13% of the time they bring in a 3rd linebacker (Jack Sanborn) instead of CB Kyler Gordon. Other than that, we're just looking at a couple snaps with different CBs.
The front 4 has more of a rotation, but it's a pretty concentrated rotation. 8 defense linemen see the field each game, but it looks like they're relying as much as possible on a 5 man rotation of the starters plus DE Darrell Taylor. Those 5 have all played well, each grading out above average for their position according to PFF. On 20% of plays a player listed as "DE" has played DT, which has helped them lean on a 5-man rotation with 3 DEs and 2 DTs. I don't have player snap counts by alignment but I think that's mostly DeMarcus Walker, who is on pace for a career high snap rate by a wide margin. Other guys are just eating a handful of snaps each game: recently acquired DT Chris Williams, rookie DE Austin Booker, and sometimes practice squadder DT Byron Cowart. They have graded poorly. Zacch Pickens took Cowart's role the one game he was healthy enough to play and did better on a small sample.
This defense looks more reliant than most on keeping their top 12 guys healthy.