The Bluebirds boss revealed that an unnamed English Premier League club tried to hijack their move for the outstanding South Korean talent last week
The 22-year-old attacking midfielder agreed to a three-year deal join the Bluebirds from Cerezo Osaka on Friday.
Mackay was full of praise about Kim, who has been compared to Park Ji-Sung in his homeland.
“This one is really exciting. Kim is going to be the next big thing out of South Korea and will win many international caps for his country," Mackay told Wales Online.
“He has the potential to be something special."
Kim had attracted reported interest from Celtic and Borussia Dortmund, while Mackay revealed Cardiff also had to beat off opposition from a Premier League who made a move for him last week.
“There was strong interest from others and a Premier League club stepped in last week but the player had already committed to Cardiff by then and we were absolutely delighted by that," Mackay said.
“We have had strong recommendations from our contacts in South Korea, including an English coach, because Kim is an up-and-coming force.”