England collapsed to 103 all out on the fourth afternoon to slump to a 405-run loss and Australia levelled the series at 1-1 on Sunday.
“There’s no getting around it, Lord’s was a nightmare for us," Pietersen said.
"Nothing went to plan for England and we were outplayed from the start.
"That said, I’ve seen people screaming for wholesale changes and blaming this and that, but everyone needs to step back and relax. We’re 1-1 against arguably the best team in the world."
Pietersen said his former team mates, particularly senior players like captain Alastair Cook, Stuart Broad, James Anderson and Ian Bell, would be desperate to put things right.
“Admittedly it was the manner of the defeat which was most damaging, but I know just how much those boys will be hurting right now and how desperate they’ll be to get back to winning ways," he said.
"Guys like Belly, Cooky, Broady and Jimmy have so much experience in Ashes series and they’ll be pressing to the lads that this series has a long way to go yet.”
Batsmen Adam Lyth, Gary Ballance and Bell are under pressure for their places following a run of low scores.
"Adam Lyth and Gary Ballance, if they weren’t already under enough pressure just playing in an Ashes series, then this next test will be that 10-fold," said Pietersen, an ambassador for Paddy Power.
"Pressure of that magnitude can make and break people, so finger’s crossed it’s the former as we simply can’t afford for them not to score big runs.”
Jonny Bairstow could be called into the side following a prolific run of form for his county Yorkshire.
“Jonny Bairstow is waiting in the wings and it wouldn’t surprise me if we saw him at some point during this series," Pietersen said. "He’s been in superb form.”
Pietersen expects Bell to prove his quality at his home ground of Edgbaston, the venue for next week's third test.
“For England to win in Birmingham our top order need to start scoring, that’s no secret," Pietersen said.
"But I expect Belly to come out with a point to prove on his home ground. He’s a class act and having batted together for many years, he’ll be doing everything to get it right come Edgbaston and if anyone can produce something special, he can.”
(Reporting by Ed Osmond, editing by Pritha Sarkar)
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