"He would have literally collapsed the deal," Tsvangirai told AFP in Stasbourg, northern France, during a rare visit to Europe.
The Movement for Democratic Change (MDC) leader also said negotiations on the power-sharing government with Mugabe must not be allowed to run on indefinitely.
"It can’t be forever," Tsvangirai said. "We cannot go on and on and on."
The MDC has said it would only join a unity government once a constitutional amendment is passed to comply with all the terms of the September 15 deal, under which Mugabe would remain president and Tsvangirai become premier.
However Zimbabwe’s parliament needs to approve an amendment to create the office of the prime minister and define its powers.
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 proposal was rejected by the opposition.