The Feyenoord star, 22, is currently in Brazil for the World Cup, and the defender has so far played a key role in Louis van Gaal's side, as his versatility at both centre back and left back has allowed van Gaal to switch up his formation and tactics with limited upheaval.
With van Gaal set to take over as Red Devils boss after the tournament, it's been no surprise to see plenty of Dutch players linked with transfers to the club, with the likes of Daryl Janmaat, Stefan de Vrij, Daley Blind, Jordy Clasie and Memphis Depay all linked - as well as Martins Indi.
United desperately need defenders after the departures of long-term central defensive pairing Nemanja Vidic and Rio Ferdinand, and Martins Indi appeared to be an ideal capture given his key role under van Gaal with the Dutch national team.
However, it may not be quite that easy to land Martins Indi, as Porto are reportedly set to make a £6.5m bid to land him this summer - and they want to close the deal to quickly to prevent van Gaal hijacking the move.
Yet the surprise move from Julen Lopetegui's men could be seen as a conscious move to snap up a potential replacement for French defender Eliaquim Mangala. The former Standard Liege defender has long been linked with a move away from Portugal having starred this season - and United are one of the teams most strongly suggested.
Mangala has previously told of his desire to play for the Red Devils, and Porto making a move to sign another physical defender could well be the signal that Mangala will be on the move. It could just be that missing out on a van Gaal favourite will be one of the best things to happen to United this summer - if they can land Mangala instead.