The full-back was a transfer target for Manchester United earlier this season.
These reports rarely seem spontaneous. At the very least they're conveniently timed during lulls in the news cycle, such as during an international break.
Yet speculation linking Tierney to a club the size of United isn’t entirely beyond reason. The 20-year-old talent should be courting such intrigue and offers are likely to come this summer.
What seems a little off, however, is the £20million figure. Although a number of factors often come in to play, and the price clubs pay for players is never entirely what they suggest to the media, there’s little doubt Tierney is worth double what has been quoted.
For a start, his rise at Celtic is almost entirely unprecedented. Although the Glasgow club have a long history of producing young players, someone of the full-back's talent hasn’t been seen in the green and white hoops since perhaps Aiden McGeady first burst on to the scene.
But alongside McGeady’s unquestionable, yet inconsistent, talent, Tierney looks far more determined, mature and resolute in his ambitions.
It's easy to forget the man from Wishaw is still young. He made his debut at the age of 17 and has since played over 120 senior games for the club. This season he also filled in as captain during the League Cup campaign. Not bad for someone who doesn't turn 21 until June.
Tierney will move on at some point, it's inevitable. He has dominated in his position domestically but has also proven, crucially, he has the quality to hold his own in the Champions League. He has two group stage campaigns under his belt in which he demonstrated he can handle the pressure and step up in quality.
When trying to decipher what price tag Celtic can demand for the full-back, the nature of the market must be taken into account in addition to the manner in which Celtic tend to sell their best players.
Victor Wanyama, Virgil van Dijk and Fraser Forster all left Celtic Park for fees varying between £10-12million, suggesting a player of their quality or promise would go for something similar.
However, three years have passed since the last big transfer deal was struck between the Hoops and a Premier League club and inflation has taken its toll.
If Tierney is viewed through the very narrow prism of a Premier League point of view, then comparisons to three players that have proven their worth at Southampton, or gone on to bigger and better things, will only add weight to the argument that a talented individual at Celtic rarely struggles to adapt to the English top flight.
Similarly, comparisons to his compatriot and fellow left-back, Andy Robertson must be taken into consideration. Although Robertson worked his way up the ladder at Hull City before a modest fee took him to Liverpool, you’d struggle to find a Premier League viewer who doesn’t believe that the £8million spent on his services are now considered an absolute steal.
Any player thought by many to be a worthy challenger to his position for Scotland should be valued just as highly.
Of course, once we’re done with the calculators, scouting reports and career comparisons to each an every player to have ever played for Scotland or Celtic, we also have to take into consideration the simple fact that the Scottish champions hold all the cards in any possible transfer saga.
Although player loyalty in the modern game is often no more than a myth, Tierney is a genuine Celtic fan and proved as much by signing a new contract this season that would see him say at the club until 2023.
Sure, long-term deals are there to be broken but only a fool would question the love and devotion this young man has shown.
Unlike Van Dijk, Wanyama or even Forster, Celtic wouldn’t be going into any negotiations knowing that, ultimately, the player will force their hand.
When it comes to Tierney, they know it would take something truly special to turn his head, not just some half-hearted offer from Southampton, Tottenham Hotspur or perhaps even Mourinho's Manchester United.
Couple that with Celtic recently reporting huge profits and a cash reserve beyond £20million, and the club does not need to sell any of its key players to balance the books.
Tierney may one day leave Celtic for a club in England or further afield, but when he does it’ll almost certainly be for a value far north of £20million. Especially if he continues improving at the rate he has shown to date.