better hope trubisky year 3 improves because it won't be the #1 defence next season.
Despite some people blindly sticking to the idea that he is shite, the stats don't really back that up these days and he was generally improving as the season went on and he became more familiar with Nagy's system and they were able to add more to it, so they could very well improve a fair bit on that side of the ball.
As far as the defence, even without Fangio, that is still a stacked defensive unit, about half of the starters are 1st rounders and a couple of the others were all-pro or pro-bowlers this season too, so as long as they are sensible with who they sign as DC (needs a similar mindset and scheme as Fangio) then they should still be just fine on that side of things too.