The Portuguese club has yet to make a decision on whether to sell the 25-year-old, while the move has been complicated by Manchester City's pursuit of Eliquiam Mangala.
While the Portuguese club considers the Argentina international as a more expendable asset than his defensive partner Eliquiam Mangala, it has yet to take a decision on whether to sell.
An aggressive though not especially quick defender, Otamendi has been at Porto since 2010, playing a part in Andre Villas-Boas' famous treble-winning team in his first season at the club.
The 25-year-old has made 16 full international appearances for his country, but has fallen out of favor since Alejandro Sabella's arrival as Argentina coach.
Otamendi's potential recruitment forms part of David Moyes' plan to add five individuals at their peak of their playing careers to the club-record capture of Juan Mata.
United's new boss has sought to attribute blame for the club's debilitating season to the Premier League-winning squad he inherited from Sir Alex Ferguson and has been granted unprecedented financial backing to overhaul it.
However, United's bid for Otamendi has been complicated by Manchester City's attempt to recruit Mangala.
Convinced it can achieve the full 50 million eurp buyout in the France international's contract, Porto has so far refused to negotiate a fee for Mangala, whom the club intends to retain for the season.
If City were to trigger that release clause, however, his sale would seriously damage Paulo Fonseca's attempt to retain the Portuguese title.