the defense will improve; I think that's certain (can't be much worse). Not sure it can improve enough to be a top-10 defense with Dame/CJ at the top of the rotation (although Portland was 8th in DR in 2018....but they had Ed Davis)
the overlooked component of defense is defensive rebounding. 3 years ago Blazers were 5th in defensive rebounding; 2 years ago they were 9th; last year, they were 27th
but then Olshey made a bet last off-season that he could sacrifice defense and rebounding in favor of offense. It was a bad bet. He might have over-corrected this off-season in favor of defense while still ignoring defensive rebounding (and post defense). The Blazers have to significantly improve their defensive rebounding over last season. Going with Roco as the PF makes that a huge challenge