The Italy international is yet to feature for United since Mourinho took charge at Old Trafford, with Antonio Valencia currently favoured at right-back.
Italian outlet Calcio Mercato reported yesterday that Blues manager Antonio Conte is plotting a January move for the out-of-favour 26-year-old.
Conte knows Darmian well from his time with the Italy national side and is reportedly keen on a reunion at Stamford Bridge.
But The Mirror claim Mourinho is unwilling to do business with his former club as he considers Chelsea to be direct rivals for the Premier League title.
GETTY
Jose Mourinho will not let Matteo Darmian join rivals Chelsea
Darmian signed for United a year ago under Louis van Gaal and went on to make 39 appearances in all competitions, winning the FA Cup in his first campaign.
The former Torino full-back has also been linked with a return to Serie A, with Juventus and Napoli also said to be interested in his services.
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