The Colombian striker has long been considered to be one of European football’s most deadly marksmen, and has netted an impressive 23 goals already this season.
At 28, he’s widely being tipped to leave Portugal this summer, and has already confessed that he’s keen to test himself in a bigger league.
Porto are open to offers of around £30m for their star man, which appears to have alerted United.
Scouts from Old Trafford watched him over the weekend, with Martinez firmly on the club’s radar ahead of the looming transfer window.
With Robin van Persie and Radamel Falcao set to be axed, Louis van Gaal is keen to lure in fresh sources of goals to support captain Wayne Rooney.
Danny Ings of Burnley and Paris Saint-Germain’s Edinson Cavani are also options.