La Liga side plan £4m raid for Celtic star this summer
Simunovic has managed to force his way back into the Celtic starting XI under caretaker boss Neil Lennon after finding himself down the pecking order towards the end of Brendan Rodgers' reign.
Celtic will be short on centre-backs going into the summer window with Dedryck Boyata joining Hertha Berlin on a free contract and Filip Benkovic returning to Leicester City following the conclusion of his loan deal.
Simunovic has been with the Parkhead side since the summer of 2016. He almost moved to Torino the following year but the transfer fell through at the last minute.