After being replaced as starter by rookie quarterback Matt Leinart, Kurt Warner acknowledged he?s considering retirement at the end of the season. ?How can you not?? Warner said Wednesday. Since Warner has a manageable contract, the Cardinals have always thought he would make a good backup for Leinart heading into the next two seasons. But speaking to Sports Illustrated after Sunday?s loss to Kansas City, Warner said that may not work for him. ?I really don?t know if I could be comfortable as a backup quarterback or if I?d go crazy in that role,? Warner said.