Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
Cagliari president Massimo Cellino has put an 18 million euro price tag on highly-rated midfielder Radja Nainggolan.
The 24-year-old has been strongly linked with a move to Serie A champions Juventus.
"I promised Radja that should an offer from a top club arrive, I would allow him to leave," Cellino said to Corriere dello Sport.
"But the offer needs to be a big one, not inferior to 18m euros."
The Belgium international scored one goal in 37 league appearances for Cagliari last season.
Nainggolan moved to Cagliari two years ago from Piacenza and is under contract with the Sardinian outfit until June 2014.