Referring to an article in the Daily Mail which quoted unspecified sources, Brazil insisted that the player would not be allowed to leave the national team squad ahead of their semi-final clash with South Korea on Tuesday.
"Contrary to the report published in the Daily Mail this Sunday, Lucas Moura will not have a medical examination in Manchester today," the Brazil federation said in a statement.
"The board have not received any request from the English club. If we do receive such a request we will not allow the player to leave."
It is believed that Moura, 19, and his club Sao Paulo have been in discussions over a move to United for the talented playmaker.
Speculation increased when United boss Sir Alex Ferguson stayed behind from a pre-season trip to Norway, reportedly to conduct club business.