Pietersen, 33, joined his team mates in Australia on Sunday after being given compassionate leave following the death of a friend.
"It was a tough time for Kev but he's a tough character," Prior told reporters on Wednesday. "He's able to ditch things pretty quickly and move on.
"You can already see in the way he's preparing, you can see mentally he's getting into it and switching on for a big series.
"He's one of those exceptional players - a guy that can single-handedly turn a game on its head in an hour or so."
England, who won the first of the back-to-back Ashes series 3-0 at home, will play two more warm-up games before the opening test of five against Australia starts at Brisbane on November 21.
While James Anderson and Broad are certain to occupy two pace bowling slots in the Ashes team, the practice matches will give the other fast bowlers in the squad a chance to seal the third and final spot.
The towering trio of Boyd Rankin, Steven Finn and Chris Tremlett were all included for Thursday's match and will have a chance to impress the selectors.
"Nets have been good fun at the moment with the big guys steaming in all playing for that place," Prior said. "The batters have been hopping around a little bit."
CLARKE COMEBACK
While Cook is sidelined to receive treatment, his Australian counterpart Michael Clarke returned to action for New South Wales with a solid 88 on Wednesday after a six-week hiatus, also due to a back injury.
Clarke, 32, skipped the ongoing one-day international tour of India to rest the injury, a degenerative condition which has troubled him since he was a teenager.
The right-hander looked rusty at the start but seemed to have no problems with his back. He was happy to spend some time at the crease before nicking one to the wicketkeeper.
"It was nice to make a few runs. I was disappointed with my shot and to get a start and throw it away... but it was nice to spend so much time out in the middle, I certainly needed it," Clarke said after the day's play against Tasmania.
"I felt completely fine. I was certainly a little bit rusty and I think my legs will certainly be a little bit heavy tomorrow but it's a nice feeling to spend so much time in the middle.
"To me there's no better training than playing a game and like I've said a few times, this is not about preparation for me, this is about trying to help NSW start the season well."
(Writing by Sudipto Ganguly; Editing by John O'Brien)
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