I think you're understating the role he'll play: I think it'll be more than the odd appearance from the bench - in a perfect world where Duffy stays injury free and maintains his form, he's still likely to come on most games, as Duffy rarely plays 90 mins.
Realistically I think, assuming he's good enough, he'll play the Brooks role; competition for Duffy and making the occasional start. He might not get as many games than if he went on loan to lower positioned side, but he will be playing in a attack-minded side, high on confidence - something which he (and those in charge of his development) would've considered.
Don't forget as well, that he's probabaly very confident in his ability; I imagine he fancies himself to break into the squad and stay there.
It's an interesting discussion though, as you're right on the face of it; he's not just going to walk into the team, as he would at other championship clubs - I can only assume the full package (style of play, manager, league position) has won him over.