Silva and Hummels are a bit similar in my opinion. One is underrated because he's stuck in France; the other one's underrated because he's half-fit and stuck at a badly struggling Dortmund side. Both of them are top three in the world in my opinion. Only Ramos is challenging them.
I think what this season had really shown, if anything, is that Hummels really is the heart of the Dortmund defence. Subotic, who some considered to be the more structured defender, has been shown up badly without him. Hummels is the organiser in addition to being the better player. Signing him right now would be akin to signing Sol Campbell back in 2001.