Casillas has done well in such an environment previously, with New Orleans. Joe Vitt had this to say about him in 2012:
http://www.wwltv.com/story/sports/nfl/saints/2014/09/03/14536962/
Casillas reported had 4.41 speed clocked at Wisconsin, and I believe he was timed around 4.61 when recovering from his knee injury in 2009. He's a fast, smart and versatile player who can do a lot of things if he can stay healthy, which has generally been an issue. He missed time at Wisconsin with knee injuries, missed 2010 with a Lisfranc injury, missed the end of 2011 with a knee sprain, and has been banged up with Tampa Bay. But in a situational/reserve role, he may be able to give us what we had hoped to get from James Anderson.
He's a decent blitzer, and he can probably play a LB/S role. Add in Akeem Ayers, and BB has nicely added to the depth at the LB position for a minimal cost.