But he added that the Zimbabwe Electoral Commission (ZEC) might also decide to delay the vote.
"It is again provided for in the constitution that it has to be done within 12 months. But as far as I can see from this, they're going to announce the run-off anytime from now."
On Friday, the MDC said it had been cheated of thousands of votes as the commission announced that Mr Tsvangirai had gained 47.9% of the votes - against 43.2% for Mr Mugabe.
The party now faces a dilemma, says the BBC's Peter Biles in Johannesburg.
It can boycott the run-off and hand victory to President Mugabe, or it can take part - under protest - in the knowledge that the violence and intimidation witnessed over the past month has created a far tougher electoral environment, our correspondent says.
Undecided
After a high-level meeting on Saturday, MDC spokesman Nelson Chamisa said no decisions had been reached so far.
MDC party rejects electoral results
"There are issues that have to be clarified," he told Reuters news agency.
The MDC says the government delayed the result for more than a month in order to orchestrate a campaign of violence to deter opposition supporters in a run-off.
Although the result of the election is inconclusive, it represents a humiliation for 84-year-old President Mugabe, our correspondent adds.
The MDC on Saturday reiterated its claim that it won the election outright.
"We are convinced that the run-off is unnecessary," said Thokozani Khupe, the movement's its deputy leader.
However she also appeared to leave open the possibility of a second round, saying: "In the unlikely event of a run-off, the MDC will once again romp to victory by an even bigger margin."
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