Personally, I have no reason to believe that Pirlo wasn't injured. I think Vieira sees Lampard as a sub at this point.
When he came on on Sunday, I was like holy shit where has this Frank been all year. Then he petered out after 10 minutes or so. I think he just doesn't have it to give for 90 minutes and he's best used for a sub, and Vieira sees that.
Not to say he won't start this sunday, but I think that would be an all hands on deck, Frank give us every drop before you retire, thing. Hope not though. If it were me, without Pirlo and Lampard, I'd play McNamara and Mendoza at CM with Iraola at CDM, I think the two CDM set is too defensive without Pirlo, and Bravo is just not as good as Iraola right now. McNamara is underrated defensively, and Mendoza has been less bad than the other options there so far, at least he can hold the ball.