If Hauritz is injured I'd send Krejza over as cover. I wouldn't say Siddle is a lock in atm, hes averaging more than Johnson this series and only slightly less expensive. Of course I wouldn't trust the averages too much as Flintoff is averaging way more than both Johnson and Siddle!
Still I could see the selectors going for more experience and going with Lee, Clark, Hilfenhaus, Hauritz. Someone said Lee doesn't want to play in the warm up to keep himself in cotton wool. If he does that then I wouldn't be picking him. He needs to prove his fitness and if he still has his form.