"The blokes have to learn. If they don't, we'll find blokes that will.
"If they're not, and making the same mistakes, then we've got to change, and that's a simple fact of cricket and results."
Set 299 to win, Australia went from 109 for no wicket to 224 all out on Monday, giving England a 3-0 lead in the five-match series.
After a strong opening stand between Chris Rogers and David Warner, the middle order of Usman Khawaja, Michael Clarke, Steve Smith, Shane Watson and Brad Haddin managed a paltry 50 runs between them in the face of inspired bowling from Stuart Broad (6-50), Tim Bresnan (2-36) and Graeme Swann (2-53).
Australia have only passed 300 once in eight Ashes innings, with Clarke at Old Trafford and Rogers at Chester-Le-Street the only batsmen to reach triple figures.
"To play for Australia, you have to perform to a level that's acceptable to everyone in our team, and also the Australian public and the media, and at the moment we're not doing that," Lehmann said.
"I think they've fought really hard and they've shown glimpses of challenging a really good side obviously, but we haven't done that consistently enough. So we'll back them as we have and we will continue to back them, but at the end of the day performances count."
The fifth and final test begins at the Oval next Wednesday.
(Reporting by Josh Reich, editing by Ed Osmond)
