I'd also argue that it wasn't just as simple as a regression for Carson Wentz.
To start the year he went 91/135 (67%) for 1,007 yards (7.4 YPA) with 7 touchdowns to 1 interception. That's a passer rating of 103. They also had a 3-1 record, and people were anointing him a franchise saviour.
The problem, however, is they had designed a really gimmicky system for him. For example, they ran a lot of bootleg intendeds on half-field reads running various depths of crossing routes. That's fine until teams start to think it's all you can do and they game plan against it. When teams did, Wentz fell off horrifically. From week 5 onwards, he went 288/472 (61%) for 2,775 yards (5.8 YPA) with 9 touchdowns to 13 interceptions. That's a passer rating of 71, and they finished off winning 4 games and losing 8. A five win season isn't bad for a rookie, but they were a much better team than that record allowed. Wentz really held them back.
Chicago is doing the same thing with Trubisky. Anyone can watch and see how many times they've got him moving out on a bootleg and hitting a half-field read who is on a cross, or an out, or a curl, etc. Fun, but that won't last in the regular season. When they took the same thing away from Wentz, he was a different QB. Not poised in the pocket, not nearly as accurate, made some really bad decisions and looked overwhelmed. That's not good for a rookie who was drafted #2 overall in massive trading up, and had spend 5 years in a pro-style system. The question becomes: Just how much more can Wentz pick up.
I don't think he regressed so much as showed that he looks wonderful with a gimmick in place, but less than stellar without it.