A constitutional amendment – which has yet to be formally put on the agenda – needs to be rushed through to create the prime ministership that Mr Tsvangirai has been allocated under the deal, along with other posts and changes.
Asked on state television when a new cabinet would be formed, Mr Mugabe said: "We will try to institute it as soon as possible."
But the opposition Movement for Democratic Change is rejecting a demand by the southern African Development Community (SADC) that the key home affairs ministry, which includes authority over the police force and elections, should be shared between the parties.
The SADC summit in Johannesburg decided that ‘the ministry of home affairs be co-managed’ between the two, its communiqué said.
Mr Mugabe, the octogenarian who has presided over Zimbabwe’s collapse, agreed to the proposal, but Mr Tsvangirai refused, saying he was "shocked and saddened" at the meeting’s conclusions.
But the MDC appears to have only two options – accept a compromise which it says does not amount to ‘equitable’ power-sharing, or walk away from the entire agreement, which it has previously said it will not do.
Appeals for intervention from the African Union or United Nations have not led anywhere in the recent past, and in any case without Mr Mugabe’s agreement – which would not be forthcoming – they would be meaningless.
Arthur Mutambara, leader of a smaller MDC faction which holds the balance of power in parliament, said that he backed the compromise, even though it was not his grouping’s preferred position, as SADC had been asked to rule on the issue and as a matter of principle its decision should be accepted.