The two clubs are reportedly keen on the Sunderland man.
The defender was deemed surplus to requirements at the Stadium of Light last season and subsequently sent out on loan to Sporting Lisbon in the January transfer window.
He has impressed in Portugal, with the Liga NOS side electing to extend his loan in the summer transfer window.
The deal reportedly includes the option to make the switch permanent, something that looks likely to happen given the performances he has put in so far.
Should they decide not to, though, it appears that Coates will have options in both Germany and Italy to choose from.
According to O Jogo, both AC Milan and Borussia Dortmund have been keeping track of the defender’s performances and are keen on his signature as a result.
Sunderland will be hoping that Sporting do not elect to make the deal a permanent one, with the Black Cats likely to be able to negotiate a good price from either Milan or Dortmund for their man.
For now, they will simply be hoping he continues to impress and attract suitors ahead of next summer.