Tierney, who is 18 years of age and plays as a left-back, is part of the Scotland squads to face the Czech Republic and Denmark in upcoming friendly international games.
Former Celtic midfielder and manager Hay has backed Gordon Strachan for his decision to call up the teenage defender.
The former Scotland international has also explained how Tierney is going to improve by just being in an international squad.
“I am not entirely sure whether or not he will start but simply being in the squad and around experienced professionals – as he is at Celtic on a daily basis – will stand him in good stead as he looks to kick on in his career,” Hay wrote in The Glasgow Evening Times.
“There is a World Cup qualifying campaign on the horizon for Scotland and it is the right thing to do, bringing in some of the younger lads who have impressed this season and getting a closer look at them at this level.”
Tierney is doing quite well at the moment and is rated as one of the more promising young players in Scotland at the moment.
The teenage defender has featured in the Europa League and in the Scottish Premiership this season, and it seems that it is only a matter of time before he becomes a first-team regular for Celtic.
Celtic are at the top of the Scottish Premiership table at the moment with 63 points from 28 matches, just a point clear of second-placed Aberdeen, who have played a game more.