Younus's injury has thrown a lifeline to struggling Imran Nazir. If he fails tomorrow he knows he won't play against India on Saturday esp. if Younus Khan (as expected) comes back into the side
As for who should be dropped for Asif, I think we do need a left arm seamer for variety's sake. So I w'd never drop Aamer (plus he has shown that he's no mug with the bat either) and it is a toss up between Gul and Rana for me
The Indians play spin better than most teams anyway, so another option will be to drop Ajmal (for that particular game) and play four seamers (Asif, Gul, Aamer & Rana) with Afridi and Malik complementing spin department