The Colombian spent several months out injured and missed the World Cup this summer after suffering cruciate ligament damage while playing for Monaco in January.
Reports claimed icing the knee was now necessary to avoid incurring further damage but Falcao posted a message on Twitter that condemns "speculation" and "lies".
"One fool speculates. A few others lacking objectivity respond and millions believe lies," the forward wrote.
"It is disastrous what a few fools can do."
One fool speculates. A few others lacking objectivity respond and millions believe lies. It is disastrous what a few fools can do.— Radamel Falcao (@FALCAO) November 10, 2014
Goal understands that United have already shaken hands on a deal with Monaco to sign Falcao permanently for £44 million at the end of the season.
The move would require the Red Devils to be satisfied with his fitness, as well as his performances, but both sides are confident that it will go through.