Deputy PM Nick Clegg says the "basic building blocks" of controversial NHS plans will remain, but changes could be made on how they work "in practice".
His aide Norman Lamb threatened to quit over the "very risky" pace of change.
But Mr Clegg said Mr Lamb agreed with him and did not want to "reopen the Pandora's Box" of the basic plan to give GPs more financial powers.
The plans would give GPs in England control of 60% of the NHS budget and let more private firms provide care.
But although the legislation has completed much of its passage through the Commons, the government has agreed to a two-month "pause" to listen to concerns - including from NHS staff - following widespread criticism of the plans - including from the British Medical Association and the Royal College of Nursing.
Liberal Democrat party members overwhelmingly rejected what they called the "damaging and unjustified" shake-up of GP services, including more scrutiny of GP consortia in a vote at their spring conference.
A petition designed to put pressure on Lib Dem MPs to make good on the motion has attracted the signatures of 1,200 Lib Dem members.
'Devil in detail'
On Sunday Mr Lamb - one of Mr Clegg's closest advisers who spoke for the party on health issues when it was in opposition - told the BBC that plans to give GPs control of budgets should not be "rushed into".
He added that primary care trusts - due to be axed under current plans - "have to stay" for a transitional period and that he would step down as Mr Clegg's parliamentary private secretary if some changes were not made.
Asked if the question of whether primary care trusts could remain was up for argument, he said: "There's no point having a pause unless you are prepared to make substantive changes at the end of it, where those substantive changes are necessary."
He said the "status quo needs to change": "Everybody agrees it is right to put more financial responsibility into the hands of GPs who know the patients best, but how you do that is - the devil lies in the detail there.
"Yes it is unusual that a government is saying look, we're going to have a pause and listen and reflect and change things where necessary, but I think it's a good thing we are listening."
He said the government was prepared to make changes if needed on issues like how GPs should be made more financially responsible "how are the consortia composed, at what period of time".
"We're not going to allow GP consortia who are not ready to take on these commissioning functions. If they are not ready by the April 2013 deadline"
But he added: "We are not saying, Norman is not saying, the Liberal Democrats were not saying at our Sheffield conference, that we are going to reopen the Pandora's Box of the basic design of a new system where you are giving GPs more financial responsibility, where you are stripping out layers of bureaucracy and you are giving local authorities, accountable to local people, much greater say in the way that the whole health system works.
"Those basic building blocks are still in place, but the details of how you make those principles work in practice are of course things we want to get right."
Speaking at the Royal College of Nursing's annual conference, general secretary Peter Carter said the reforms as they were designed at the moment were a real risk to the NHS.
He said he supported the basic principles of giving patients more power and involving NHS staff more in decision-making.
But he added: "Despite the honourable principles behind the bill, it could well turn out to be the biggest disaster in the history of our public services.
"The sheer size of the changes must not be underestimated. Even during the good times, when the sun was shining and the money was flowing, these reforms would have been huge. Now that the clouds of economic gloom hang over us and the cupboard is empty reforming the NHS like this begins to look very difficult indeed."