He tends to run a base 3-4 but he does run a lot of 4-3 fronts. Sometimes those 4-3 fronts have 3 down linemen and one stand up edge rusher in what would normally be a DE position with his hand on the ground in a normal 4-3. He's certainly a multiple fronts guy that will use different things to attack different offenses.
For instance, Fangio, who is Staley's mentor, more or less ran a 5 man front against us when we played the Eagles this year to shut down our run game.
I think this could give new life to Mathieu. He's still very smart and should still be good at crashing down on the LoS to add a run stopper to the light box that Staley tends to use. But, we will need to add some good young Safeties soon.