Hansi Flick has opted to bring in Spain captain Rodri over the summer which has slid Casado further down the pecking order at the Estadio Camp Nou.
The La Masia graduate had looked set to join Saudi Pro League outfit Al Hilal on a permanent transfer, but that move has stalled, and all parties are now looking at other options.
Manchester United had been linked with a possible offer, but that now looks unlikely, after the Red Devils landed Carlos Baleba for £70M.
Reports from Spanish outlet Marca claim Besiktas and another unnamed Turkish club have now reached out to Barca and his agent Jorge Mendes has opened talks.
The main barrier to a sale appears to be Barcelona's request for a €25M transfer fee which falls outside the budget of several interested parties - but if lowered, there could be renewed links to the Premier League for the 22-year-old.