Tory MPs, along with a few Labour ones, are queuing up to say he cannot possibly master the brief because he hasn’t got a university degree – let alone an economics one – and left school at 15 with no qualifications.
Others point out, with a wry smile and raised eyebrow, that he used to be a postman, as if this somehow renders him incapable of being chancellor.
It’s true that Johnson’s poor knowledge of economics will be his Achilles heel. Just as Osborne, a history graduate, struggled to land blows on Gordon Brown and Alistair Darling, Johnson won’t be able catch his sparring partner out as much as he’d like.
But Labour’s best economist, Ed Balls, would have been a toxic choice; the Tories were desperate for Ed Miliband to give him the job.
Aside from his rather crucial weakness, Johnson will be a formidable opponent. His time as home secretary passed without a hitch, with none of the disasters that destroyed his predecessors, as did his swift ascent through the ministerial ranks. A former trade unionist, he is loved by the left, but is as Blairite as they come.
He is also undoubtedly the Labour party’s best media performer, with a knack of making the most ridiculous policy sound entirely reasonable.
Johnson does, of course, have flaws. He follows the Ken Clarke approach to hard work, believing there is more to life than politics. That’s fine when you’re in government, with armies of officials to support you, but opposition – something Johnson has never known – is a tough slog.
The Tories used to have sleepless nights at the thought of Johnson challenging Brown, and leading Labour into the election. They can’t now pretend that he’s not a serious threat.