I can't think in terms of a first XI these days, I think the game's changed so much, you have to think of at least 14 equally. Subs seem to be a genuine part of the tactics more than they used to be, and a player coming on for the last 20 minutes is no less a player in my eyes - it's just a strategy to use them in the last 20 minutes with fresh legs, or a calming influence etc.
I think there's an old school train of thought was sees subs are 'inferior' to the first XI, but those days are long gone now. Its a natural perception, but I'm not sure it's a correct one.
That's why I think players like Delph have a hugely significant role to play even if they aren't superstars. To get a great 30 minutes out of someone like Yaya, you might need someone like Delph to take over some of his burden. Or it may be that Delph plays for an hour then Yaya comes on as an impact sub (hypothetical example). Neither work without the other though, and that's what people forget.