Speaking at a fringe event at the party's annual conference in Brighton, Heidi Alexander said the proposals agreed by NHS England and the government were 'fanciful'.
'The question of the finances that are available to provide health and care services in this country is my number one priority,' Ms Alexander told the Health and Care Forum meeting.
'We've got a situation where the present government have said that they will find £22bn of efficiency savings over the next five years. Personally, I think that is fanciful,' she added. 'I don't know how they are going to achieve that level of efficiency savings without compromising patient care.'
NHS efficiency savings
NHS England's chief executive Simon Stevens agreed to find £22bn of savings to help plug the expected £30bn funding gap by 2020. The government pledged to fund the remaining £8bn a year. But many health service and policy leaders believe the target is unachievable.
Ms Alexander also agreed to look into the costs to the NHS of litigation. The costs of medical negligence, Ms Alexander said, was 'a sign of failure' in the NHS. 'One of the best ways we can go about dealing with this is to provide safer care. So we do need to bring that bill down,' Ms Alexander said.
The new shadow health secretary also called for social care funding to be addressed urgently. 'Cuts to social care are cuts to the NHS by a different name,' she said.
'How we pay for care for the elderly is one of the biggest unaddressed questions.'