The 25-year-old celebrated his first start for Cardiff following his move from Sunderland with a goal in each half to ensure their promotion challenge continued with a 2-1 win over City.
And Mackay believes Campbell is still building his fitness after a lack of games prior to his move, with the medical staff keen not to overburden the player.
"I am delighted for him, you could see the quality there," said the Scot. "You could see the intelligence, the strength and the link-up play. And for him to get the two goals is obviously really pleasing for him.
"More than anything I am delighted about the lack of ego about the boy and the way he has thrown himself into everything that we do here. He has bought into it and he has grasped it.
"I felt it was time to start him and it is about bleeding him in gently. He has not played a lot of football in the last few months and we have to treat him properly as he builds up to match fitness.
"We knew he was going to have to come off the pitch, his whole body cramped up in one big go!
"I don't doubt the type of footballer we have got, it is about making sure we get him into the team in the correct way."