The 25-year-old marked his first start for the Bluebirds following his move from Sunderland with a goal in each half in the 2-1 win over Bristol City to keep his new side's promotion bandwagon rolling on.
The opener, a smart finish from Tommy Smith's clever through ball, broke the resistance of a stubborn Bristol City side in first half added time.
Campbell then secured the points from inside the box after being found by Aron Gunnarsson's huge throw.
Mackay feels Campbell is still building his fitness after a lack of games before the move to south Wales and the Scot explained his medical staff are keen not to overburden a player who had not previously started a match all season.
"I am delighted for him, you could see the quality there," he said.
"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 in to everything 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."