Deputy Prime Minister Nick Clegg has defended Danny Alexander's suitability to be Chief Secretary to the Treasury.
There have been questions about whether Mr Alexander has the economic background for the job he moved into after David Laws quit over expenses.
The Lib Dem leader said Mr Alexander was "as determined as anyone else" to tackle the economic "mess".
He urged people to judge the Lib Dem minister - and Tory Chancellor George Osborne - after the forthcoming Budget.
Mr Alexander was switched on Saturday from the role of Scottish Secretary to become number two in the Treasury, where he will play a crucial role in negotiating spending cuts with other government ministers.
Formerly Nick Clegg's chief of staff, Mr Alexander, 38, played a key role in negotiating the coalition agreement between the Conservatives and Lib Dems.
This committed the Lib Dems to backing more than £6bn of spending cuts this year, something they had opposed before the general election.
However, some have questioned Mr Alexander's suitability for the job of leading the government's spending review this autumn as, unlike Mr Laws and several other Cabinet ministers, he has no specific economic experience and has not worked in the City.
Former Cabinet minister and Labour leadership contender Ed Balls described the appointment as "highly curious".
But Mr Clegg said he was the right man for the job and the "sad" departure of Mr Laws did not "alter one bit" the government's determination to tackle the deficit.
"The partnership between George Osborne and Danny Alexander is an excellent one," he said. "They will now set to work to fill this black hole so we have sustainable public finances and we can do the good things that this coalition government wants to do in the years ahead.
"I think Danny Alexander, George Osborne, in fact the whole government, should be judged by the Budget later in June and also, of course, by what happens to the state of the public finances in the years ahead."
"We have inherited a terrible, terrible mess from the outgoing Labour government and Danny Alexander is as determined as anybody else - he is extremely gifted as well - to get to grips with this financial crisis once and for all".
Mr Alexander also got the backing of senior Conservative MP Michael Fallon, tipped to become the new chairman of the Commons Treasury Select Committee, who said some of the most effective chief secretaries of recent times did not have financial experience.
While "some understanding" of the City was necessary, he said the role was essentially a "political job".
"You don't have to have a degree in economics to do the job well. You need very good judgement, you are being asked every day to take decisions, to refuse spending requests.
"You need an understanding of Whitehall. You need the ability to persuade your colleages to survive on a little less money than ideally they would like."
Mr Fallon said he was sure that Mr Alexander would be "sensitive" to the wishes of the Conservatives in his approach to cutting borrowing.
"I think you have - as part of the coalition - to have a Lib Dem in that key role and that makes perfect sense as there are very difficult economic decisions to be taken."
The BBC News Channel's Chief Political Correspondent Laura Kuenssberg said there was unease in Conservative ranks about the number of ministerial jobs given to the Lib Dems - including five seats in cabinet - and Mr Alexander's appointment fed into that.
However, she said Mr Alexander was already a cabinet minister before being given this key job and concerns about his age - he is 38 - should be seen in the context of the fact that the chancellor is also in his 30s.