The Nottingham Forest defender and seven of his old Dons team-mates flouted the Covid protocols to go out in the city last August.
They had just lost 1-0 to Rangers when McKenna, Bruce Anderson, Craig Bryson, Sam Cosgrove, Michael Devlin, Jonny Hayes, Matty Kennedy and Dylan McGeouch all decided to go for a meal.
One player tested positive for coronavirus, with boss Derek McInnes left furious.
Then First Minister Nicola Sturgeon publicly shamed them on TV.
McKenna, who turns 24 on Thursday, has since joined Forest in a £6million deal.
But he’s revealed for the first time how the episode affected him, as he vowed to learn from the huge mistake.
McKenna, who could start for Scotland against Serbia on Thursday, said: “That was a far more difficult time for all of us than people probably realise.
McKenna wants to become a hero with Scotland Credit: Kenny Ramsay – The Sun Glasgow
“The gaffer was very frustrated for the first couple of days after it, and rightfully so.
“Our actions left him without eight players.
“We couldn’t even train and do our jobs because of our stupidity.”
McKenna is full of remorse for breaching the regulations, but has revealed what actually happened that day.
He added: “There were only meant to be three of us going out for food.
“But you know what it’s like in a dressing-room after a game. People hear what you are doing, and ask if they can come along.
“It was a bad idea going out after we’d lost to Rangers anyway. That didn’t help matters.
Aberdeen players flouted Covid protocol to go out in city last August
“But we weren’t intending for eight of us to go out.
“People just asked what we were doing, and we said we’ had booked a table for dinner and suddenly others were joining us.
“We didn’t think anything of it, but we should have done.
“That’s what happens normally, and we just completely forgot the responsibility we had to stick by the government guidelines and club protocols on coronavirus.
“When you look back, people may not think it’s too bad. But with the seriousness of everything that was going on it was a very bad error of judgement.
“We all got tested on the Tuesday and seven came back negative, and one was positive.
“That’s when it hit us we might be in a bit of trouble.
Aberdeen boss McInnes was furious with actions of his eight playersCredit: PA:Press Association
“It was at the time when Nicola Sturgeon was doing her daily press conferences which everyone watched.
“You would have thought we were the only people who had ever broken the rules.
“That was difficult to take because it drew so much attention to us.
“But then that’s part of the job of being a professional footballer, and we just had to accept it.
“You need to watch what you are doing every single minute in every single day and we didn’t do that.
“So we had to deal with the consequences. But we have all learned from it. Footballers often attract attention for errors they make, but the majority bounce back.
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“It’s a massive test of character being in that kind of situation but you must handle it.
“We were the ones who put ourselves in that situation so we had to deal with it.
“You can’t let it get you down. You have to bounce back from it, get back on the pitch, and let your football do the talking.”
It was a huge learning experience for McKenna, who is one of the most professional and conscientious players in the game.
He missed the last Scotland triple header due to injury but he’s been flying for Forest, and ready for action in Belgrade, in what would be the biggest game of his career.
McKenna said: “I’m just really excited to be back in the squad. I didn’t play in the last three games and the boys did brilliantly. But if I’m called upon for this game then I’m ready.
“I’ll always be ready to play for Scotland.
McKenna has been warned about Serbia and Fulham hitman Mitrovic
“We have a few lads at Forest who were at Fulham last year — Harry Arter, Anthony Knockaert and Cyrus Christie — and they warned me about Aleksandar Mitrovic.
“They have made me aware of just how good he is.
“The problem is they have all said we can’t even give him one chance or he’ll score!
“That’s not exactly what you want to hear when you are a centre-back.”
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McKenna is loving life at Forest, and even scored his first goal in a year last week in the win over Coventry.
He added: “I need to score more, to be fair. It’s something I need to work on, to be more clinical in the opposition box.
“I tend to get my head on things, but the ball goes over the bar or it’s flicked wide. But it’s always a great feeling to score a goal, especially as a defender. That feeling you get is just brilliant.
“Only problem is, I never know what to do to celebrate!”
McKenna felt forced to hand in a transfer request at Pittodrie last summer, as he feared he wouldn’t get his dream move to England.
He said: “It was a difficult one. I didn’t know if Aberdeen were being fair with the offers they were getting.
“The money was probably more than they had ever been offered for a player before, and they were knocking it away like it was nothing.
“I realised how big an opportunity it would be for me at the time.
“Maybe the transfer request was a bit of frustration on the back of that.
“But I think everyone at Aberdeen stuck by me at that point. The gaffer backed me and told me not to worry.”
News source: thescottishsun