The Blancos legend believes that the careless nature in which the Argentine forward is being looked after could lead to their fierce rivals tempting him to the capital
Transfers between the arch-rivals are rare, with Javier Saviola the only player to have moved between the two clubs in the past 14 years when he left Barca for Madrid in 2007.
Sanchez, who starred up front for Los Blancos between 1985 and 1992, suspects that the Argentine is not pleased with his current deal, which runs until 2018 and contains a buyout clause set at €200 million (£164.7m).
"Messi is not feeling valued highly enough by Barcelona with his contract and they must be careful," he told BeIn Sports.
"If Barca are careless, I can see Real Madrid signing him."
Messi has been with the Blaugrana since 2000 and has been awarded the Ballon d'Or four times at the age of just 26.