Lehmann's side trail 2-0 after Sunday's defeat in the second test at Lord's, meaning Ashes-holders England can clinch the five-match series when the sides next meet at Old Trafford on August 1.
The lacklustre Australians have been out-played for much of the series and Lehmann admits they deserve all the criticism currently coming their way.
Australia captain Michael Clarke also made no attempt to hide the fact that his team are struggling when assessing the 347-run loss at Lord's, which came hot on the heels of defeat in the first Test at Trent Bridge.
With the Ashes in danger of slipping away, Lehmann will use the three-day tour match against Sussex at Hove, starting on Friday, to look at potential changes to the team for the third Test.
He has pledged to give reserve wicketkeeper Matthew Wade and spinner Nathan Lyon the chance to impress against Sussex.
Jackson Bird, James Faulkner and David Warner, who will return from Australia A duty in time for the third Test, have also been told they could be needed as the selectors scramble for a winning formula.
"I say all the time the whole 18 have a chance to play," Lehmann said.
"I'm not ruling anything out.
"That is what happens when you have two losses, every option is open.
"As selectors Rod (Marsh), John (Inverarity) and I will have to speak about it and see where we come to.
"Ashton (Agar) is a bit sore and I don't think he will play in Sussex so Nathan Lyon will play obviously.
"Then we just have to look at the wicket (at Old Trafford) because history shows it spins a bit.
"If it's going to spin we will certainly have a look at two spinners. Wade will play in the tour match too."
England have already banked three centuries in the series, two to Ian Bell and a maiden Ashes hundred to Joe Root, but Lehmann conceded Australia's batsmen have proved far too flimsy.
"We have certainly got to bat better, we know that," Lehmann said.
"It's probably more of a mental thing. We need to make sure we're ready to go...have a good time down in Sussex, play some decent cricket and bat a lot longer than 90 overs.
"We had two guys get to 50 at Lord's but 50 isn't going to get us to where we need to go.
"Clarke and (Usman) Khawaja batted really well but they need to make hundreds when they get to that position, just like young Root did."
Australia's tour has been marred by a series of off-field issues, including the sacking of former coach Mickey Arthur just 16 bays before the Ashes and reports of a rift between Clarke and opener Shane Watson.
But Lehmann has told his players to stop looking for excuses.
"We actually need to get some results because that would stop all the talk and make people realise we are going the right way," he said.
"We can't worry about what anyone else says or writes. We accept the criticism, we deserve the criticism to be fair."
Meanwhile, England's only problem is the fitness of Kevin Pietersen, who was unable to take the field in Australia's second innings after suffering a left calf strain.
England batting coach Graham Gooch admits the selectors will look at potential replacements for Pietersen because they can't guarantee the South-African born star's availability.
"Whether he will be fit for Old Trafford...he'd be in doubt," Gooch said.
"I'm sure there will be some cover supplied."
However, asked if Somerset's Nick Compton – dropped ahead of the Ashes to allow Root to open in his place alongside England captain Alastair Cook – was the most likely candidate, Gooch added: "I'm not a selector, so I couldn't give you an indication to whether he would be the next cab off the rank or not."