United sent scouts to watch Martin at the start of the season with the transfer window still open to run the rule over the left-back.
Mourinho was keen to bolster his options on the left side of his defence with Luke Shaw still hampered with injury problems.
But a deal never materialised for Martin, who recently earned a call-up to Spain’s under-21 squad at the start of the year.
Real Madrid, Barcelona and United’s arch-rivals Manchester City have also been linked with the 20-year-old in the past.
And El Gol Digital claim the Red Devils will reignite their interest and are willing to meet the £27million release clause in Martin’s contract.
The report suggests Madrid have dropped out of the running to sign Martin after they brought in Theo Hernandez during the summer transfer window.
Barcelona are reportedly still interested in signing the full-back.
GETTY
Jose Mourinho is ready to activate Aaron Martin's release clause
However United are said to be in the driving seat with Martin favouring a move to Old Trafford.
Martin only made his professional debut in October last year but has already established himself as one of Spain’s young promising stars of the future.
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