Mackay attempted to take France international Capoue to Cardiff City Stadium, and they got as far as agreeing a fee and personal terms with the player and his then club Toulouse.
But Capoue, has been ruled out of Sunday's game by injury, stalled over committing to Cardiff, and eventually chose to move to White Hart Lane when Spurs entered the race for his signature.
Mackay quickly moved for Chile international Medel, making him the club's record signing in a deal worth £11million.
Medel has made a very impressive start to life in south Wales, excelling in the win against Manchester City and the goalless draw with Liverpool.
Mackay admits he was disappointed not to have landed Capoue, but has been delighted with the addition of Medel.
"When one door closes another opens," he said.
"That's why our recruitment department work the way they do and have done for three years.
"Me and (head of recruitment) Iain Moody worked towards having this, where if one door closes you can go after another one. 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.
"You look at levels of players and see who is a realistic target. A situation arose quickly and Iain acted quickly. We managed to bring in Gary.
"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."
Cardiff have made an assured start to their first Premier League season, picking up two draws and that eye-catching success over Manchester City.
Mackay's side have been renowned for tactical discipline and work ethic, but they have shown they are not lacking in technical ability.
But the Scot does not pay attention to the view of their style of play from outside the club.
"That's not something I particularly care about to be honest," he said.
"It's about myself and the players and my coaching staff - it's about their inner belief and competing in the Premier League.
"We are under no illusions about the size of this task and the quality of the teams we are facing.
"It is about knowing we can compete against them every game and we have a belief in what we do. It is about doing it over the course of a season. This group has a focus on an end game."