George Sabra added, however, that it would make any hope of a peace deal more difficult, according to the BBC.
Syrian forces backed by heavy air strikes have seized more than a third of rebel-held territory in the city.
Rebel factions say they have pulled back to a more defensible front line.
Thousands of civilians have fled besieged districts of Aleppo after heavy fighting over the weekend.
Mr Sabra, chief negotiator for the High Negotiations Committee (HNC), told the BBC World Service that the loss of Aleppo would "not be the end of the revolution".
"Aleppo is an important place for the revolution but it's not the last place," he said.
"Right now, we have so many places under the power of (the) Free Syrian Army."
He warned that the military campaign being waged by the Syrian government and its allies was "killing a part of the political process".
"Nobody can think about peaceful solutions in these circumstances," he added.
Read alsoDW: Syrian regime routs rebels in eastern AleppoThe advances by pro-government forces followed two weeks of relentless aerial bombardment and captured all northern districts held by the rebels in eastern Aleppo.
The Russian defence ministry said rebel factions had lost 12 districts, amounting to 40% of their territory.
Analysts say it is a devastating setback for the rebels, who seized the area in 2012. The government has made steady gains in Aleppo since Russia intervened to bolster President Assad in September last year.
Ibrahim Abu Al-Leith, a spokesman for the White Helmets rescue group in eastern Aleppo, said there was "mass displacement and morale is in the gutter".
