Cardiff had agreed a fee and personal terms before Capoue elected to move to Premier League rivals Tottenham instead.
Mackay promptly made Chile international Gary Medel his club's record signing in a deal worth £11m - and he is delighted with the midfielder's progress so far.
"When one door closes another opens," Mackay said.
"That's why our recruitment department work the way they do and have done for three years.
"Our recruitmnent system were well down the line with Toulouse, finances had been agreed, but we knew he was a top quality player. We had various options in that position if we didnt get him.
"I have been pretty open in that we looked at Medel for most of last season. At one point we didn't feel we would get anywhere near him because the season before AC Milan were involved.
"Capoue has gone to Spurs and did well up until his injury. I had no doubts he would - he is a top French international.
"Likewise we have a top Chilean international who I am delighted to get. I look at him every day in training and, like with Steven Caulker, we had two top players come into the football club. It has raised our standards again."