If I recall correctly from reading the summaries of Fangio's defense that many have posted, his most important position is OLB.
His OLB's need to be able to both rush AND cover because they make their own reads and react based on the offensive alignment, so his OLB's need to be field smart as well.
My impression of Fowler is that he's 90% rush and any cover help you get is just a bonus. Matthews maybe fits that mold. Haven't seen enough of the rest of our OLB's to hazard a guess.