The college chairman, who steps down in November, said there was 'clear evidence' that if consortia were not big enough, many would fail.
But he called for failing consortia to be offered support or merged with neighbouring groups, instead of allowing private firms to take over.
'I don't see the problem with asking for specific help... but we must resist the millions of pounds given to consultancies for spreadsheets that aren't even part of the treatment.
'We must do everything we can to resist private companies taking over consortia.'
Professor Field said he 'welcomed the philosophy' of GP commissioning set out in the White Paper, although he said 'the devil will be in the detail'.
'It's hard to argue with the principles of clinically-led commissioning,' he said.
'But one of my biggest concerns is that the best managers leave and we have to buy them back from the private sector.'
Another problem will be getting practices that have never worked together to share data and commission as a team, he said. Professor Field assured GPs that their new role would not cast them as the villains of the system as NHS budgets contract.
'GPs will not be the new NHS managers. Look, the Daily Mail doesn't like us and we've worked on that but you'll never stop it, because sensational public health stories sell newspapers. But we must be brave. We are more likely to sort out the worst (of the NHS) with GPs in charge.'
It will be important to look after new GPs and encourage partnerships to ensure a new generation of leaders emerges, said Professor Field.
Professor Field also believes revalidation will benefit from PCTs' demise, with 'responsible officers' likely to be placed within each consortium and less management interference.