United have been strongly linked this summer with Juventus' Vidal, the 27-year-old who featured for Chile at the World Cup.
Quoted by El Mercurio, a newspaper in Vidal's homeland, Sampaoli made reference to a potential 80 million US dollars (around £47million) deal between Juve and United being jeopardised by the knee issue.
Vidal underwent surgery on his right knee in May before returning to help his country reach the last 16 in Brazil.
Sampaoli was quoted as saying: "The problem in Vidal's transfer from Juventus to Manchester United for 80 million dollars is the doubt that Van Gaal has with respect to the improvement of Arturo's knee."
You'd think those 50 fake medicals he had would have proved he was healthy. Guess its not enough to convince this guy making up another rumor.- Synergy