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.
"I am delighted for him, you could see the quality there," said Mackay. "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 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!"
The opener, a smart finish from Tommy Smith's clever through ball, broke the resistance of a stubborn Robins side in first half added time and Campbell then secured the points from inside the box after being found by Aron Gunnarsson's huge throw.
Sean O'Driscoll's men did grab a consolation in added time when Ben Nugent scored a bizarre own goal, heading the ball past David Marshall under no pressure, but they were left to reflect on a third straight away defeat since the former Nottingham Forest boss took charge.
And it was Campbell's second goal, a close-range finish from a long throw, which was especially frustrating for O'Driscoll.
"If we hadn't conceded a poor second goal we could have walked away with a point which would have been extremely pleasing," he said. "The second goal was disappointing because every day in training we say we cant concede poor goals in this division. We didn't deal with that as well as we should have.
"They are a good team and with the front two of Campbell and Bellamy they deserve to be a good team. Their movement and interchanging caused us problems. But credit to the lads they stuck in and stuck together and everyone has reacted positively to any negatives in the game."