He also stresses the uncertainty of the amount of money that will be raised by anti-avoidance measures and the increased bank levy.
'Not a bottomless pit'
And he says that there will now have to be additional austerity as a result of the chancellor's pledge to balance the budget by the end of the next parliament without raising taxes.
"There's even more austerity than we'd expected because the chancellor has decided he wants to continue for an additional year in order to get the books into surplus - he doesn't need to do that for his own fiscal rules," Mr Johnson told the BBC.
Much of the attention on the Autumn Statement has focused on the decision to raise the state pension age to 68 in the mid-2030s and 69 in the late-2040s.
Mr Osborne told the BBC: "The reason we do this is because our country is getting older and we want to go on being able to afford really good pensions for people. There is not a bottomless pit of money."
"The alternative is a pension system that would collapse because it was unaffordable," he added.