Jacas was listed as a LB when drafted. That may be his role in the base defense, with him rotating to the edge on passing downs.
A base defense made up of our best 11 looks like this:
Landry - Barmore - Williams - Jones
Spillane - Jacas
Gonzalez - Woodson - Byard - Jones - Davis
I wonder if LB will be de-emphasized in the base defense, with Jacas used in a hybrid role much like Vrabel was.