Watson, 34, was dismissed lbw twice in Cardiff as he has been many times in his career, raising question marks about the all-rounder's place in the side for the second match at Lord's starting on Thursday.
“Every time the Australians get beaten, it’s always Shane Watson who takes the heat and surprise, surprise, this time it’s no different," Pietersen said.
"Let’s make no bones about it, he’s got to stop getting out lbw and start scoring some big runs, no-one is un-droppable after all, but he’s a match winner."
Watson's place is under threat from Mitchell Marsh who scored two centuries in Ashes warm-up games.
"I’ve seen first-hand what damage Watson can do and for me, talk of dropping him is massively premature. He needs to be backed by the Aussies, not hounded,” Pietersen said.
Pietersen said the form of wicketkeeper Brad Haddin, who will not play in the second test for family reasons, and batsman Steve Smith were a bigger worry for Australia.
"My concern, if I was an Australia supporter, would be the performances of Brad Haddin and Steve Smith," Pietersen said.
"Haddin scored 29 over his two innings, hardly an invaluable contribution, while Smith only scored a handful more runs than Watson, yet he’s supposed to be their dangerman and the form player in the world."
Smith failed to pass fifty in Cardiff following a brilliant run of form which made him the top-ranked batsman in the world.
"He was totally overshadowed by (England's) Joe Root, who looked on a different level to him," Pietersen said.
"Rooty showed just what a class act he is and I wouldn’t be the least bit surprised if at Lord’s he picked up right where he finished off in Cardiff.”
Pietersen thought the England bowling attack was also by far the most threatening in Cardiff.
"Just as vital for the Aussies as their batting woes, is their misfiring bowling attack," said Paddy Power ambassador Pietersen.
"Contrast their performance to that of the England attack, who bowled beautifully. It was great to see all of our bowlers putting the Aussie batsmen under sustained pressure and thankfully they got the rewards they deserved."
(Reporting by Ed Osmond; editing by Toby Davis)
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