Speaking to Chilean newspaper El Mercurio on Tuesday, Sampaoli - who has extended his contract as Chile's coach until the 2018 World Cup in Russia - claimed United manager Louis van Gaal is concerned about Vidal's recovery, after having knee surgery in early May.
Vidal underwent surgery in the hope of fixing a persistent meniscus injury once and for all but that decision cost the 27-year-old midfielder in Brazil, as he struggled to produce his best football as Chile were eventually eliminated in the round of 16 by the host nation.
United have been heavily linked with the Juventus man throughout the close-season and Sampaoli said the reason no transfer has been completed is down to Van Gaal and Vidal's right knee.
"Right now the great obstacle to his $80 million [£47 million] transfer from Juventus to Manchester United is Louis van Gaal's doubt over the improvement of Arturo's knee," Sampaoli said.
Vidal joined Juventus from Bayer Leverkusen before the 2011-12 Serie A season and has been a part of the Turin-based club's three Scudettos in succession.
The man from Santiago has played 96 Serie A matches for Juventus, scoring 28 goals, while he has 54 caps for Chile, hitting the back of the net eight times at international level.
Vidal played in three of Chile's four matches at the World Cup but failed to complete each of those games.
He is currently on a pre-season tour of Asia with Juventus.
