Eliaquim Mangala is a tough tackling central defender who is enjoying a particularly impressive spell with Porto.
The 22-year-old Frenchman began his career in Belgium with Standard Liege before switching to Porto in 2011.
Mangala may be known to English football fans for breaking the metatarsal of Arsenal defender Kieran Gibbs in tackle in a Champions League match for the Belgian side.
Now at Porto he is enjoying an excellent season, and when United's scouts went to watch him in action in early February he scored in a 4-0 win over Guimares.
The centre-back has made 30 appearances for his club this season and has been an ever-present in the Champions League, and will hope to stand firm when Porto travel to Malaga to defend a 1-0 first leg lead in a fortnight.
Milan lost Thiago Silva to PSG last summer and are keen on a big name replacement, and it is claimed that Italian superagent Luciano D'Onofrio, who has good relations between Porto and Milan, can help close a deal.
D'onofrio is the former vice-president of Standard Liege, Mangala's former club, resigning in 2011 the same year the defender was sold. As an agent he has dealt extensively with Milan, involving Marcel Desailly, and also counted former Porto goalkeeper Vitor Baia in his portfolio, playing a part in his transfer to Barcelona.
It is claimed D'Onofrio, who has a suspended conviction over financial allegations against him dating back several years, has made initial contact with the player's representatives on behalf of Milan, being asked because of his Liege connections.
Rumours this week have also suggested Manchester City are interested, they have reason to search for a defender with Joleon Lescott expected to leave in the summer and doubts also existing over Kolo Toure's future.
Mangala is yet to win a cap for France but it can't be far away, but the defender may consider that he needs to make a move to make that step.
How concrete United's interest is remains to be seen. It will largely depend on the future of both Rio Ferdinand and Nemanja Vidic, and he won't be the only potential centre-back signing on the club's radar.
But he is one of the best options on the table, providing the Red Devils are willing to commit upwards of £15 million, which is uncertain if they are planning a mega move to bring back Cristiano Ronaldo.
For AC Milan, their resolve to dip into the market and spend the cash will largely depend how their existing defenders get on in the Nou Camp when they defend a 2-0 first leg Champions League. That will be the acid test.
Would Mangala be a good signing for AC Milan?