Mackay, who has made only one signing so far this summer, said: "Cornelius has great potential and is a very good capture for the club. We tracked him all last year. But by Christmas he was in such good form we really didn't think we had much chance because top clubs in Europe were watching.
"We were in competition internationally with another club and then three Premier League clubs moved in to try and take him as well. But we were able to do things quickly and I have to give a special mention to Copenhagen. They were tremendous - efficient, professional and friendly."