The 18-year-old has been one of the Scottish Premiership side's strongest players this term.
The 18-year-old has been one of the Hoops' most impressive performers this season despite only making two appearances for the first-team before the start of the campaign.
And with the left-back also working his way into the Scotland side, and arguably putting in a man of the match display on his debut for the national team, Tierney's emergence has surely been the biggest positive from a mixed season for the Bhoys.
And while McGrain - who made over 400 league appearances for the club and won seven Scottish Premiership titles during his playing days - has been impressed by what Tierney has achieved this term, he suggested what he needs to work on to take his game to the next level.
"Maybe it's because he is so left-sided that he is now seen only as a left-back. But that's okay because, let's face it, he hasn't done too badly," he told The Scottish Sun.
"My right foot was alright but my left foot was dreadful because I hardly used it.
"I'd urge him to work on his right foot, but I'm sure Kieran will. Maybe also his heading."
While Tierney has undoubtedly been one of the most pleasant surprises for the Hoops this season, he arguably faces a more difficult challenge next year than the one he had to contend with this term.
After such a strong campaign, the academy graduate will now have to deal with the pressure of such high standards being expected of him on a consistent basis.
But McGrain is clearly confident that by working on the aspects of his game that need improvement, Tierney could be the one with multiple title medals under his belt that the future crop of youngsters look to for advice.