Inexperienced DBs, injured safeties, and some marginal DL and LBer play lead to giving up big plays. Listen to Jeff Woody's podcast and then rewatch the game. You can see that the LBers were trying to fill inside because the DL was getting pushed around in the middle. Williams did a great job of bouncing a couple carries to the outside, and the safety did not respond quick enough. Lewis had a tough game and got sucked in to try and help in the running game. Him matched up on Williams is always going to be a bad deal. The fact that officials absolutely refused to call holding on KU really help them a lot Saturday. It was pretty bad.
I suspect that Elsworth is about 65-75%, and Lewis is also still injured, but playing. Generally I would blame a lot of the big plays given up against OSU and KS as poor fit by the LBers and safeties.
If the ISU DB and LBers could catch anything this team would probably undefeated right now. That is so frustrating.