United chief David Moyes has played Mata, a £37.1million capture from Chelsea, out wide since his arrival in January.
United favourite Hill, who made 134 appearances for the club from 1975 to 1978, told the Daily Star Sunday: "I can't understand why Juan Mata is playing on the left wing.
"This little guy is playing all over the place at the moment.
"He is lost in games, roaming, not picking up players and getting caught with the ball when he has it - it is not his best position."
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