Charlie Mulgrew raised his eyebrows as he saw a fearless 17-year-old charging into tackles at Celtic's Lennoxtown base.
And his first impression of Kieran Tierney on his maiden training session with the Hoops first team was: "Fair play, wee man."
The left-back rose to prominence in Brendan Rodgers' all-conquering side before a club record-shattering £25m move to Arsenal in 2019.
But when he began mixing it with the big boys Mulgrew was still a Parkhead player, before his move south to Blackburn Rovers in 2016.
Now back in the Scottish Premiership at Dundee United, Mulgrew told the Scottish Sun : "People talk about KT's attitude now he's an Arsenal player. But I saw that on the first day he trained at Celtic. Honestly, he was fearless.
"I know what it's like to make the step up from training with the youth team to the first team and it's terrifying.
He was only 17 at the time and it's a daunting thing to do, especially when you've been at the club since you were 12 or 13 and have looked up to all the players in the first team.
"My little boy is in the Celtic academy now. Whenever he sees a first-team player he's starstruck because it's the guys he normally sees on the TV.
"We've all been there, growing up and watching your heroes. I was there myself, watching Henrik Larsson, Chris Sutton and John Hartson and I suddenly found myself in that environment.
"Personally I was a bag of nerves because it's surreal and almost unbelievable that you find yourself in that position.
"But KT settled into it a lot quicker than I did. He just started firing into people! I remember thinking to myself, 'Fair play to you, wee man!'.
"Don't get me wrong, he was a lovely boy and really respectful. But he wasn't scared to get stuck in by closing people down and making tackles.
"It was his first training session, but he treated it like his last.
"These days, a lot of young boys come into the first-team training and they don't think it's cool to run about and work hard. They won't want to appear too keen or busy.
"But KT was the complete opposite and he stood out because of it. I can genuinely remember saying to myself, 'This boy will do for me', because his attitude was just brilliant.
"He didn't care about who he was up against, he was just sprinting everywhere and giving it absolutely everything.
"We were doing boxes and he was smashing into tackles all over the place, really intensely pressing the ball. That's what Celtic were all about."
