United boss Louis van Gaal name-checked Vidal twice in his press conference after a 1-1 draw with Sunderland on Sunday.
The Juventus star has been linked with United all summer but club chiefs are worried about his right knee.
Vidal had an operation before the World Cup to repair his meniscus and was never fully fit in Brazil. He failed to complete any of the three games he played.
According to the Independent, United are considering an appearance-based contract similar to the one offered to Michael Owen.
The 27-year-old will go through a rigorous medical at the Carrington training ground before any possible deal is done.
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