And there are fears that his youthful supporters could take to the streets in violent protests.
With one third of the polling stations reporting, Ms Johnson-Sirleaf was up by 20 points after a second round vote on Tuesday, according to preliminary results released by the National Elections Commission yesterday.
Full results are expected within a week, to be certified by November 23.
But Weah, who has traded soccer spikes and jerseys for stylish loafers and a white suit in his bid to become the 23rd president of Africa's oldest independent republic, alleged ballot tampering and ballot-box stuffing, while also accusing the NEC chairwoman Frances Johnson Morris of "prejudicial" conduct against him.
"While we were preaching transparency and democracy, our election was fraudulent," Weah told reporters at a hastily called press conference at the headquarters of his Congress for Democratic Change (CDC), as hundreds of party supporters milled around on a dusty pitch outside, breaking into cries of "No Weah, No Peace" at every opportunity.
"We have substantial evidence of vote fraud," he said.
In the possession of the CDC is a stack of ballots pre-marked with a black thumbprint in the box for Ms Johnson-Sirleaf, the official NEC stamp on the back, which Mr Weah showed to the pack of international and local journalists, who have been covering his fairytale campaign since early this year.
"These are papers that shouldn't be out here, they should be in the ballot box," he said of the ballots, numbering more than 100, that were given to the CDC by "concerned citizens".
Another 3,000 ballots were making their way to Monrovia, his media spokesman Steve Kweah said.
Tuesday's run-off was the culmination of an electoral process forecast under a peace agreement signed to end the second of Liberia's two civil wars since 1989. An October 11 first round gave Weah a 28.3 per cent win over 21 other candidates, including Johnson-Sirleaf, who was nearly ten points behind.
By rights, he should have won in a landslide, as the popular hero of the Liberian youth who make up more than half of the country's population of three million people, many of them veterans of the gruesome conflicts which have reverberated around troubled west Africa.
A campaign rally before the first round drew crowds of more than 100,000 people. Many had marched for hours before waiting under scorching sun to hear their idol speak his message of reconciliation and inclusiveness.
He also secured key endorsements from his vanquished opponents and newly elected members of Liberia's bicameral legislature going into the second round, among them Prince Johnson, new senior Senator for northern Nimba county, notorious for having lopped off the ears of president Samuel Doe on camera before killing him in 1990.
But in the final two weeks of the campaign, Johnson-Sirleaf spent lavishly, hiring a helicopter to take her to the most remote parts of the interior and waging an aggressive media blitz that underscored her qualifications and hammered away at Weah's lack of formal education and political inexperience.
Stepping back from the bellicose rhetoric that has marked much of his electoral campaign, Mr Weah said yesterday win or lose, what was important was peace and stability for a country torn apart by war.
"Whether I win or not, we are talking about transparency, peace and nation-building," he said, urging his supporters to remain calm. "It's not about me winning, or somebody winning, it's the whole process."
Concern that his followers will not heed the message of calm was by last night gripping the capital Monrovia, whose one million citizens are weary of the conflicts that have left the city without running water or electricity, decent roads or adequate sanitation. Jonah Cooper, a 19-year-old Weah devotee, too was ready to move forward -- just as long as his hero was the one inheriting the presidential mantle. "Weah will take the mansion and all will be well," he said. "If he does not, if that old lady is there, this country will be divided and destroyed, and back in war. We will not allow that to happen. I will die before that happens."