Judging from comments here people are really really concerned about a defense that for 3/4"s of the regular season ranked #1 in run defense and yards per carry and it's a real concern. When Spillane and all the defensive front players were healthy, this was the best run defense in the NFL.
And people here keep suggesting Ellis and Spillane weren't good in coverage. Be more specific... when Ellis was in man coverage on TE's, he was great at it. But team zone coverages were crap because they were often out of spot or in the wrong zone entirely. This was true in the backfield also, where we had arguably the best CB trio in the league.
Kevin Byard was brought in to QB the backend of the defense, he will set people up in the right place and make sure they're running the correct zone.
Spillane is your SAM middle LB, Ellis is your WILL LB'er behind him playing in space. This when healthy is a really good duo. Of course they need the DL in front of them to stay healthy and play well. That's a scheme dependent thing, but true of all LB'ers. Vrabel just happens to run a system where all his LB'er are small mobile LB'ers and he allows his front to do the heavy lifting.
Chad Muma, Namdi Obiazor and Declan Williams are the depth. Muma at least is a great athlete, Namdi Obiazor and Declan Williams good athletes. As Spillane and others have proven, ILB is one of those positions where great athleticism isn't as important as instincts, timing and a willingness to get dirty. The least athletic among them could end up being the best player.
This is a pretty good group, plus they have some OLB's with some flexibility now. I don't understand the concern assuming health.