One play that jumped out to me against Michigan State was Ian Williams's tackle for loss where he met the runner in the hole and drove him backwards. Where had that been to this point? What did ND do differently on that play?
As you can see, the difference is really only that Ian was able to keep his feet on his stunt and help fill the hole that had opened up and allowed for a cutback. Compare the first play in this video, which is the very first play that MSU ran on Saturday with the second one, that happened a bit later in the quarter.
If ND can get more production like the latter out of Williams, the run defense will likely improve noticeably. However, perhaps an even bigger issue is the play of the middle linebacker. Re-watch the video and this time keep an eye on Toryan Smith (#49). On both plays there is a pretty big hole for the back to cut back into, and on both plays, Toryan overruns the hole slightly and just isn't in position to make the tackle. It will be interesting to watch him against Purdue and see if he is able to feel out these holes a bit better and not wind up diving against his momentum trying to make an arm tackle on the passing back. If he isn't, how long until Te'o starts to get more snaps on non-passing downs?