The former skipper is delighted at the teenager’s progress after being discovered by his dad John as an eight-year-old in Wishaw.
Tierney was the one bright spot for Celtic in Sunday’s Scottish Cup shoot-out flop and he is already being linked with clubs across the border such as Manchester City, Liverpool and Southampton.
“I never had a left foot, though! I’m sure he wants to enjoy as many years there as he can – enjoy success and win trophies.
“He is living the dream, so I hope he does that for as long as he wants to do it. If he moves down the line, then that’s the way football is. That’s a decision he will make. He just has to learn the game.”
McStay admits Tierney was one of the few Hoops players who got pass marks at Hampden.
He said: “Hopefully, he is allowed to mature as a player at Celtic. It’s a great environment to learn the game. His performance on Sunday suggested he had been playing there for a few years.
“In terms of pressing, Kieran is probably the only one who led the team.
“I was sitting with my dad and my brothers Willie and Raymond in the stand close to where Kieran was playing in the first-half and sometimes he had two or three players round about him.
“But he was working so hard to close down and I didn’t see much communication round about him.
“He was left to do it himself, but he handled it well. He looked to be tiring, but he still found energy to get up and create a goal in extra-time.
“It was my dad who identified Kieran at a very young age so for us, from a family point of view, it’s great to see him progressing.
“Playing with one of the biggest clubs in the world is not a bad place to play your football.
“Hopefully, next season he will get the chance to play against some of the best players in European football.
“There are aspects of his game that I’m sure he will want to work on, but right now he is one of the star players.”