Berra has hunger for Wolves' Premier bid
While hosting a coaching session at Meadowbank Stadium, Mackay was overwhelmed by the athleticism of a scrawny teenager who skated through every task presented to him. He then received an unexpected insight into the player's desire and dedication, attributes which would help him progress to captain Hearts. "I remember one of our summer coaching sessions, which were for some young Hearts boys and other players we represented," said Mackay.
"Christophe came along, he was about 15 or 16 then but I didn't represent him at the time.
"We did a session of six 220s, which are heavy runs, and he was absolutely cruising.
"He was a shy lad and he said to me, 'Gary, could I maybe stop now. I have my PE exam at school tomorrow'.
"Of course, he should never have been doing that session with a PE exam the next day, but that showed the desire the lad had.
"I'd never seen him play but I knew his athletic qualities and spoke to Bert Logan, my business partner.
"It took a wee while to get him on board as my client but he has developed into a quality footballer. I'm sure he'll be able to handle playing at Wolverhampton.
Berra is now represented by English-based agent Mark Donaghy, having severed ties with Mackay late last year.
Donaghy helped finalise the 24-year-old's 2.3million transfer over the weekend, although the foundations were laid last summer when Mackay negotiated with Hearts and Wolves as the Midlands club's 1.5m offer for Berra was submitted and then rejected.
"It's a disappointment to me the way things have happened because I dealt with Wolverhampton last summer relating to Christophe," he continued.
"If he had gone somewhere else I'd probably have less resentment and rested easy. But, having worked hand in hand together for five or six years and built a relationship on trust, somebody who will know him less as a person has done a deal which was already in place.
"That doesn't stop me hoping Christophe goes on to have a good and prosperous career. Hearts needed the money.
"I think they needed to sell somebody and Christophe probably wanted to go.
"I had always said to Christophe that the summer was a better time to move club. That's my opinion for any player, not just him.
"In January you have to adjust quickly to a new environment and games immediately, whereas in the summer you have pre-season games to settle in.
"I don't think there will be any chance for Christophe to bed himself in.
"I felt last summer that he would benefit from another season in a Hearts team that was doing okay. Christophe has always had a desire to get to the very top level.
"He'll be a wee bit down that nobody from the Premier League has taken a bite at him but I think he should take great confidence that Mick McCarthy, a guy with years of experience playing in the same position at the top level, has continued to covet his services."
With Wolves currently top of the Championship, Berra's quest for England's top flight is progressing according to plan. Mackay, pictured below, has no doubt he is destined to play Premier League football.
"In the fullness of time that will happen because of his own single-mindedness and resolve," he continued. "He has been through a hell of a lot at Hearts, as have a lot of the young boys.
"Everybody goes on about players having mental toughness now and I think Christophe has gained in that respect in a perverse kind of way. You want to be in a stable situation but, unfortunately for Christophe and Hearts, there hasn't always been stability.
"We've seen salary problems and prior to that the Riccarton Three escapade, there's always been something going on.
"He handled the captaincy well and we used to speak after virtually every game.
"That's the trust element that I think you want to have with players. Unfortunately, somebody turned his head and that trust element was broken. I hope he goes on to have a very prosperous career. I have a wee tinge of resentment towards the agent who stepped in but that's the kind of game we're in."
Given the number of clubs with Premier League ambitions hovering around the Championship, there are those in England who regard the second tier as equally demanding as the first.
"I speak to John Colquhoun (former Hearts team-mate] a lot and we discuss the teams down there because John spends a lot of his time in England," said Mackay. "You have teams like Swansea, managed by Roberto Martinez, who used to be at Motherwell, playing really good football.
"John is a fan of Doncaster's style of play, then there's Cardiff with young Scottish lads like Ross McCormack in the team.
"It is a highly competitive league. The middle to top of the Championship is probably as competitive as middle to bottom in the English Premier League."
Mackay watched Berra thrive on competition throughout his teens and into his 20s, whether it be a sprinting session, a PE test or an SPL match. The strictest examination of his credentials for England's Premier League is yet to come but few would bet against him passing with flying colours.