From what they've shown so far, they are overall a worse defense than they were last year.
With the loss of Betiku, Illinois has virtually no pass rush from their D-Line and they don't blitz much as they can't cover in the back end one on one.
Their best best overall player on Defense by a wide margin is MLB Jake Hansen. The rest of the front 7 though is not good. Besides not having a replacement on the line for Betiku, it doesn't look like they have a replacement for Harding at linebacker - and the guy that they probably thought might be that replacement in Milo Eifler looks like he's regressed.
*not sure it has any bearing on the reason for that regression but Eifler was one of the few players who was quite outspoken about not wanting to play this season.
It doesn't help that their Outside Linebackers and Safeties take awful angles too often and don't have the speed to recover.
They were expected to have a very solid (though incredibly thin) secondary this season with Hobbs, Adams and Smith being joined by RSFr Marquez Beason - their highest rated recruit in the Lovie Smith era as a legit DB rotation. But Beason has been a complete non-factor and the rest of the DB's are having to make a ton of tackles in the 2nd level because their front 7 outside of Hansen can't do anything and thus they end up getting burned deep pretty regularly.
The one thing they do well (especially Hansen) is create turnovers. It is almost like either they get a turnover before you reach the redzone or they aren't stopping you from scoring. Minimize turnovers and you win pretty easily against them. Otherwise you end up in a dogfight, just like we saw last year.