The Red Devils were strongly linked with the Argentinian over the summer, but David Moyes elected against sealing a deal for his services during an underwhelming window for the Premier League champions.
But, the Old Trafford side have maintained an interest in Garay, and have had scouts monitoring his progress ahead of a potential winter return.
But, Napoli chief Rafael Benitez is also keen, and may rival any offers for the 27-year-old.
The former Chelsea and Liverpool boss is believed to have the funds at his disposal to force through an arrangement, and is keen to beat his former enemies to his signature.
Garay is highly-rated in Lisbon, but a £17m release clause may leave Benfica powerless to keep their centre-back.
Moyes is thought to see the South American as a perfect long-term successor to the aging Rio Ferdinand, and was initially plotting a move at the end of the current campaign.
But the Scot may have to push his plans forward to see off Benitez.
Could Ezequiel Garay be a Manchester United hit?
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