Exactly 12 months earlier Mackay had watched the same player score twice against Watford at Glanford Park as Scunthorpe earned a draw.
Hooper was virtually unknown outside of The Championship when Neil Lennon splashed out £2.5m to bring the striker to Glasgow but after eight free scoring months in Glasgow the striker is making people sit up and take notice.
The loss of Robbie Keane left a big gap to be filled but with 13 goals from 16 SPL matches the 22-year-old has done as much as anyone for Celtic’s title push.
Sunday’s double against Rangers was the product of a striker bursting with confidence and came no surprise to Mackay who has been keeping tabs on Hooper for three years.
“I remember speaking to Nigel Atkins (the then Scunthorpe manager) after he had sold Martin Paterson to Burnley for £1m,” the Watford boss recalled. “That was in 2008 and he had Gary earmarked as Paterson’s replacement.
“I suggested that losing Paterson was a bit of a blow but he said he had a someone coming through that would be a top player, young Hooper.
“Gary was someone that I kept an eye on but there was never any chance that he could have come to Watford.
“We did well signing Danny Graham who was coming to the end of his contract at Carlisle but Scunthorpe protected Hooper with a good contract. Nigel knew what he was worth and that a club like Watford wouldn’t have the finances to make a move.”
Joining Celtic is about more than having the ability to pop the ball in the net on a regular basis.
The demands of the supporters and the stakes involved in a two horse race can get the better of some players.
Mackay played alongside Pierre van Hooijdonk, Paolo di Canio and Jorge Cadette and knows as well as anyone the qualities needed to be a success.
“Some people can’t handle playing in front of 60,000 fans,” the Watford boss added. “There’s a lot of expectations as a Celtic player, being in the spotlight 24/7 can be quite disturbing for some people.
“It only takes a bad game or two and people can be on your back, if you have the attitude to handle that it makes a big difference.
“Gary looks like he’s enjoying playing for Celtic, his touch for the first goal against Rangers was sublime. He scored with the air of someone confident about their football.
“He’s improved since his transfer. At Scunthorpe he was a potent threat, someone that was always sniffing about the shoulder of defenders and looked like a player that was definitely going to a higher level.
“He’s had to get more involved in the build up play, with Celtic he’ll see a lot more of the ball and he’s adding things to his game.
“With all respect to Scunthorpe when you play with better players it helps your game and I can see that in Gary.”
Despite the excitement around Hooper comparisons with Andy Carroll are premature according to Mackay.
Hooper outscored the £35m Liverpool striker last season in The Championship but according to the Watford boss there’s a lot more involved in a players value than just scoring stats.
“I wouldn’t compare him with Andy Carroll,” Mackay added quickly. “In terms of their career paths they are totally different players. Gary started at Grays Athletic, then to Southend and onto Scunthorpe.
“Andy Carroll came through the Newcastle Academy and has always been well regarded. He’s came through that apprenticeship and that’s different to Hooper’s upbringing.”
Fourteen years ago Mackay was in the thick of the derby action but enjoyed a more detached view of Sunday’s clash.
Sandwiched in-between Championship fixtures with Reading and Bristol City the Vicarage Road boss was able to enjoy a couple of hours away from the demands of his job.
He added: “Being a distance away from things I could sit and watch the game in relative calmness. If it’s a win I can enjoy it, if it’s a defeat my own result is still going through my head.
“I keep my eye on how things are going in Scotland and Sunday was a fantastic result for Neil.
“He’s got the team playing very well but no-one will know better than him that there’s a long way to go and that both teams still have everything to play for.”