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The 27-year-old underwent surgery in May in the hope of fixing a persistent meniscus injury once and for all, with his fitness subsequently hampered during the World Cup.
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Vidal met with his agent and senior officials in July to discuss the possibility of a transfer to Old Trafford and Sampaoli says the reason no transfer has been completed is down to Van Gaal.
"Manchester United want Vidal, but Van Gaal has some doubts," Sampaoli told El Mercurio. "Right now the great obstacle to his $80 million transfer from Juventus to Manchester United is over the improvement of Arturo's knee.
"Vidal set a great example at the World Cup in Brazil by playing when he wasn't in perfect shape."
United has been chasing Vidal for much of the summer, with Juventus chief executive Giuseppe Marotta adamant the Chile star would only be sold if he expressed a desire to leave Turin.
Vidal has previously insisted he is "not going to Manchester", while new Juventus head coach Massimiliano Allegri has ruled out an exit following the player's return to preseason training.