Johnston grabbed the second goal as Celtic breezed past Falkirk to reach the last 16 of the competition.
The win saw several of the players reunite with former Hoops goalkeeper Scott Bain after he made the move to Falkirk this summer.
Bain was a favourite among teammates in the Celtic Park dressing room, but Johnston thinks he had one trick up his sleeve that caught the ‘keeper out.
Alistair Johnston reacts to his goal against Falkirk
It’s a rare occasion to see the Canada international on the scoresheet, despite displaying plenty of great attacking traits as a full-back.
His effort was a beautiful left-footed curling shot and Johnston joked that using his weak foot was enough to catch Bain off guard.
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He told the Daily Mail: “It was kind of just a left-foot swing of the pendulum and I think I caught Bainy out.
“I was joking with the lads — I don’t think I’ve ever actually taken a shot with my left foot before.
“So, I’m guessing he’s never seen it in his two-and-a-half years that I played with him.
“I’m glad I caught him off guard and managed to sneak it in there.
“I haven’t seen him yet, but I’ll definitely be giving him a little bit. He deserves it. He was having a go at my garden saying that my grass was all patchy — he’s right and I know he’s got a great garden.
“I’ve got a little bit of a thing going on there, me and him. He’s a good lad.”
Alistair Johnston could reach Celtic appearance milestone this season
Johnston is almost certainly one of the best Celtic right-backs of the 21st century and definitely the best since Mikael Lustig.
Forever the model professional, the 26-year-old has always been available and always reliable.
So far, the right-back has made 114 appearances in a Celtic shirt, meaning that there is every chance that he could reach the milestone tally of 150 appearances before the end of the season.
He would need to feature in another 36 games for the Hoops and it feels like that is a given due to his quality across the pitch.