Pietersen, labelled by Australian captain Ricky Ponting as cricket's next superstar, is one of the game's big entertainers and McGrath says his wicket will be prized in the five-Test series, which gets underway at Brisbane's Gabba ground on November 23.
"Without any doubt he will be one of the keys alongside 'Freddy' Flintoff for England in this summer's Ashes campaign," McGrath wrote in his newspaper column for The Sunday Telegraph.
"Because of the way Pietersen plays, it's a bit like (West Indian) Brian Lara, you always feel as though you are a chance to get him out."
McGrath, who is Test cricket's all-time leading fast bowler with 542 wickets in 119 Tests, says playing under pressure will be Pietersen's biggest examination under Australian conditions.
"Coming out and batting without any fear when the boys in front of you have put some runs on the board is one thing," McGrath said.
"But coming out and batting without fear and playing your shots when there isn't as many runs on the board is another.
"I'm sure his team-mates would not be happy if Pietersen played a big shot and was dismissed cheaply when the pressure was really on. But, given that he loves to play strokes from the outset, that's always a chance."
McGrath, who has made it a habit of nominating his 'bunny' ahead of a Test series, said Pietersen, who has scored five hundreds in his 18 Tests, is a wicket he is chasing in the Ashes series.
"Is he a target of mine leading into the Ashes? Absolutely," he said.
"Everyone in the England team is a target but particularly their class players. At the end of the day, my job is to get rid of their openers and expose the middle order."