"He would have literally collapsed the deal," Mr Tsvangirai said in Strasbourg, northern France, overnight during a rare visit to Europe.
The leader of the Movement for Democratic Change (MDC) also said negotiations on the power-sharing government with Mr Mugabe must not run on indefinitely.
"It can’t be forever," Mr Tsvangirai said. "We cannot go on and on and on."
Earlier, Mr Tsvangirai said his party was not turning its back on its pledges and it was vital for Zimbabwe to seize the chance to form a government.
"We are not walking away from the deal; we support the deal. We continue to defend the deal. But it must be a deal that reflects equitable power sharing," he said.
"If we miss the opportunity, then the tragedy for the country is even too desperate to contemplate."
Zimbabwe’s economy has collapsed with the world’s highest inflation rate and major food shortages, and many see the power-sharing deal as a way out of the economic and political crises.
The MDC has said it would join a unity government only when a constitutional amendment is passed to comply with the terms of the September 15 deal, under which Mr Mugabe would remain president and Mr Tsvangirai become premier.
Zimbabwe’s parliament needs to approve an amendment to create the office of the prime minister and define its powers.
Mr Mugabe vowed to soon form a new government after regional leaders proposed earlier this month that his ZANU-PF and the MDC immediately set up a new cabinet. The opposition rejected the proposal.
EU development commissioner Louis Michel said the opposition leader retained the European Union’s backing for his quest to become prime minister.
"I assured Mr Tsvangirai of our support. Under the current circumstances, he doesn’t seem able to assume his responsibilities (as premier), he wants the means provided by this responsibility, and we support him in this," Mr Michel said.
"The strongest pressure that the EU can apply" to help him "is to announce a very strong (aid) commitment if a government is soon formed".
The European Commission provided Zimbabwe with about €90 million ($170 million) in humanitarian aid last year, but all development aid to Mr Mugabe’s regime has been frozen.