Marshall played well in place of Lavar Arrington at weak-side LB last year after Arrington went down with an injury. He is some 15 lbs. lighter than Pierce so he may be asked to pack on a few pounds during the offseason. Pierce also moved inside in 2004 after having played exclusively OLB, so one would have to think that Gregg Williams can figure out a way to make it work.
One important factor is that Marshall just signed a three-year contract.
I guess that makes him a Redskin for Life.