The 23-year-old midfielder impressed in his first season in the Welsh capital as the Bluebirds stormed to promotion and the Championship title.
"I think Kim is absolutely ready to make the step up to the Premier League," said Mackay.
"He's an international, very comfortable on the ball and just desperate not to give it away.
"He is certainly a very exciting player, that was the reason I bought him in the first place. I think it was a case of us getting a player that ordinarily we might not have managed to get.
"Early on at Cardiff it was very much about him settling in. The lad knew he had to learn the language and he knew he had to learn the culture.
"There was also just getting used to the league itself, the physicality of the Championship. The relentless nature of it."