Instead of who? I love Mascherano and he is amazing but if Puyol is fit, Masch doesn't have a place in the starting line-up. Maybe he'll come on as a sub for Busquets but I doubt that will happen if Busquets and Puyol are both fit.
say we're leading 2-0 or 1-0 and pep wants play conservative (which he shouldn't since it has come back to bite us in the ass so hard against arsenal and real last week) he could take off someone (perhaps misfiring villa) and play Mascherano and Busquets together rather than bringing on keita, or maybe even sub puyol for mascherano.
Anyways on the whole these past couple of months we have seen what a valuable signing he has been especially without abidal and puyol, without him we would have been screwed from Feb-till now.