Didn't Pattinson get injured and Siddle was cooked after the Adelaide Test. That's why they had to change the attack.
I can't remember who the 3rd bowler was that Summer but yeah the rotation policy under Mickey Arthur was a sham and Pat Howard was a rugby toff who was so far up James Sutherland's arse hole it wasn't funny. How that bloke lasted so long in a crucial role was ridiculous
Selectors should have picked Hazlewood instead of Hastings that test match who was 12th man.
Pattinson was ruled out injured, Siddle and Hilfenhaus (he's the other paceman you were thinking of) were rested due to the rotation policy. In addition, Shane Watson came in for Rob "nine" Quiney as the 4th fast bowling option at the WACA.
So, Pattinson/Siddle/Hilfenhaus were replaced with Johnson/Starc/Hastings. The first time in 127 years (1885) that the Aussie test team changed their entire frontline pace attack between tests! Terrible timing to make such a drastic change leading into a test series decider, let alone a short 3-day turnaround between tests.
Hastings bowled one good ball that test (which turned out to be the only test he would ever play) and that was the ball that got AB De Villiers out. Beautiful outswinger IIRC. Otherwise, he was too slow and pedestrian to use at the WACA, conceding nearly 6 runs/over in the 2nd innings.
Now that you mention it, yes, Hazlewood was in the squad and if the selectors were definitely going to make changes for the WACA test, he should've been selected over Hastings. Although only 21 at the time, he was already destined for big things, being in the Aussie U19 World Cup team at age 17, making his state debut for NSW also at age 17, and ODI debut at age 19.
As a result, Hazlewood missed out and had to wait a couple more years to make his test debut.