It'll never happen. The UFC would never sanction this fight, and they wouldn't allow Silvia to go fight him in PrideFC.
Why? Because Silvia would lose.
Why would he lose? Because Aleksander is one of the best strikers in the world of MMA. In my opinion, his striking is either a little worse or a little better than Cro Cop's. His ground game might suck, but it has to be better than Silvia's.