Sissoko was actually a very good defensive midfield player. A good wrecking ball. Problem is, Rafa seemed to play him as a box to box midfielder (or some hybrid attacking-defensive mid, or defensive-attacking? i was never sure) and alongside Mascher, or a sitting Alonso, Momo's techincal deficiencies were there for all to see.
Lucas isn't a very good defensive midfield player. In fact, he's not a very good player (period). But hey, to be fair to the lad, it's not his fault he's a first teamer.