The Daily Star says United have been eyeing up Fernando Torres' former Atletico Madrid strike partner for some time.
But Atletico insist it will take an offer of £40m to trigger talks for the Spanish League's Player of the Year.
The Argentine, nicknamed 'El Kun' after a Japanese cartoon caveman, has flourished since Torres left Spain to join Liverpool last summer, scoring 20 goals last season.
His agent, Jose Segui, admitted yesterday: "I have received an offer from an English club who have asked for it to be passed on to Atletico Madrid.
"I can't say anymore until I talk to the club about the offer. I can only say that we are talking about one of the best teams in Europe, but not Chelsea."