Valencia and/or Young are guaranteed to start as Kagawa will probably lack the sharpness required to play at this level and Moyes will want 'discipline' in a European away game, even if it is against Olympiakos. Fellaini is fresh back from injury, so I'd expect Fletcher to be used for his defensive prowess and work rate in midfield, along with experience in big European games. Ferdinand is next to be dropped to the bench if any of Rafael, Jones or Evans are fit.
I agree with everyone saying that we should stick to the same tactics from the Palace game (look through my posts and you'll see that I've said it numerous times myself). Our front four should be Rooney, Mata, Januzaj, van Persie starting every game possible, while if one of those cannot play for whatever reason, Kagawa is the first to come in rather than Valencia/Young (who should be used as subs during games to change it up, but this is should be our plan B, not A). However, I will sympathise with Moyes and don't think it is the worst move to be a bit cautious away in Europe, as long as he's still positive enough to at least set the team out to score a goal or two. We're almost guaranteed to see this, though:
de Gea
Rafael Smalling Vidic Evra
Fletcher Carrick
Valencia Rooney Young
van Persie
Bench will include the likes of Ferdinand, Fellaini, Kagawa, Januzaj, Hernandez... so it'll be a strong bench to try and nab a goal if we don't get one with the starting line up. Quite fancy another 0-2 win here.