Considering he was almost guaranteed to aggressively target David Harris in free agency, I don't think he is comfortable with who he has, and their defensive ranking from last year doesn't matter much considering they'll have an entirely different scheme this season.
Point is, Rex is on the record saying he likes the depth they had in their linebackers.
Additionally, the ranking was trotted out to show that even without Alonso the LBs and defense as a whole are talented enough to be highly rated.
Sure, scheme may change, but most teams run hybrids anyways. Denver was a 4-3 last year, but ran plenty of 3-4 looks, as well as some other packages. I'd imagine that Rex's defense is no different.