The Slovenian played five times for Cardiff and scored one goal.
He came off the bench to play for City last Saturday at Bristol City and was then on the bench for Newcastle at Arsenal last Monday.
His loss doen't help a City squad missing the injured Rudy Gestede and Steve McPhail who is not quite fit enough to return to action yet.
"Newcastle have picked up injuries in the last week which means he now has to be on their bench," said Mackay.
"We obviously wanted to keep him.
"He is a talented individual who is going on to do very well in his career.
"Sometimes these things happen and Newcastle had three injuries that they didn't legislate for.
"Haris is a terrific young lad to have around the place. He is going to be a real talent."
"He was someone that [Newcastle manager] Alan [Pardew] felt would benefit from coming in here and picking up Championship experience for his career.
"Alan and myself go back a while and we are good friends. It was something he highlighted that he hoped wouldn't have to happen.
"Because personally he wanted him here and he wanted him to be part of our squad. Haris himself was desperate to stay until the end of the season.
"Because he felt it would improve his game no end and I think there were signs that was beginning to show."