The Daily Mail says Eagles manager Dougie Freedman has swooped for the Israel international on a six-week deal and the 27-year-old will go straight into the squad for Saturday's south London derby against Millwall.
Palace and Cardiff were both keen to keep a lid on the deal until it was ratified by the Football League.
But Keinan jumped the gun after posting on Twitter: "Yeh! On loan to crystal palace for 6 weeks. Can't wait for the first game!!!"
However, his Tweet infuriated Mackay after the Bluebirds boss was informed of the player's post.