Olivier Blanchard said the Budget in March would be a good time for Chancellor George Osborne to "take stock" of his plan A.
The comments, in an interview with BBC Radio 4's Today programme, came after the body trimmed its forecasts for the UK and global growth.
The British economy is now expected to expand by 1% rather than 1.1% this year, and 1.9% rather than 2.2% next year.
There are fears that official GDP figures for the final quarter of last year, due to be published tomorrow, could show a contraction, adding to pressure on Osborne to ease his approach.
Blanchard said it was clear the world was "not out of the woods yet".
"We've never been passionate about austerity. From the beginning we have always emphasised that fiscal consolidation should be slow and steady," he said.
"We said that if things look bad at the beginning of 2013 - which they do - then there should be a reassessment of fiscal policy," he said.
"We still believe that. You have a budget coming in March and we think that would be a good time to take stock and make some adjustments."
He added: "We think that slower fiscal consolidation in some form may well be appropriate."
"I agree with Blanchard when he says we need to be steady and sensible and pragmatic, not dogmatic not ideological, in trying to balance the books," he told London's LBC Radio.
"And to be fair to us as a Government, I think we have been much, much more pragmatic than some of our critics (say).
"They said that we are just blindly marching along slashing things left, right and centre.
"Actually, when we realised it was going to take longer to balance the books, we said OK, the rules, the deadline we had originally set were going to be pushed into the future."
Clegg pointed to the higher-than-expected leap in public sector borrowing in December, revealed in official figures."If tax receipts go down and welfare payments go up, we are not going to just keep chasing our tail. We are going to allow borrowing to go up to cover those costs in a sensible and pragmatic way."