Campbell is now leading the line for Cardiff in the Premier League.
Mackay told Absolute Radio: "Well, he's someone that I remember coming out of Man United and going into Hull as a loanee a long time ago, and did really well at Hull. I really liked him from them. Had an injury at Sunderland, came back from it, and then was available for a year, but through circumstance didn't play and wasn't involved. When we knew that we could get him for - because there was only six months left on his contract - we got him for £600,000, Ian. I think it was an absolute steal.
"He's got power, he's got pace, he's in his mid 20s, he's played for England. There's a confidence about him. I've told him that if he keeps going the way he's going there's a chance that he forces his way back into the England squad, because there's not a huge pool of players to pick from. And I do think that he's got the power and the pace to cause people problems. And I think you saw that against Man City."
Mackay was speaking on Absolute Radio's Rock n' Roll Football Show with Ian Wright. Saturday's from 5pm.
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball