The Scotland international defender moved south in the summer but injury has restricted his initial impact at the Emirates Stadium.
Hartson, though, believes the youngster will prove to be a shrewd addition for the north London outfit and says he already surpasses compatriot Andy Robertson, a Champions League winner, in some aspects of his game.
“Also, average players are going for £30m or £40m and Kieran is certainly not average.
“Celtic could have held out for another £5m or so but it is still good money they have banked, especially due to the fact he has come through the ranks and hasn’t cost the club a penny.
“He is worth £25m and he can excel in a different league because there’s a perception the Scottish league is just not as good.”
Tierney’s appearances for Arsenal to date have been limited by a hip injury, with the 22-year-old having featured only twice for the starting XI and still waiting on his Premier League debut.
Nevertheless, Hartson believes he will be a hit once he gains his fitness.
“Once Kieran finds his feet he will do well. Nobody wants to go to a new club injured and not to be playing,” he explained.
“Kieran is a proud boy and he will be wanting to prove to everyone what a good player he is.”
Indeed, the former Wales international believes that Tierney is stronger in some aspects of the game that compatriot Robertson, who has won widespread acclaim at Anfield since joining the club in 2017.
Fellow left-back Robertson was nominated for the FIFA FIFPro Men’s World11 in a defensive slot but was ultimately beaten to a position, finishing seventh overall.
Hartson says that Tierney can have a similar impact and claims that the new Gunner has attributes that eclipse those of his Scotland international team-mate.
“People talk about Andy Robertson. I would say at this moment in time he has better delivery but Kieran is a better defensive player,” he said.
“Kieran gets low and defends well and he can also play as wing-back if needed. I watched Robertson when he was playing for Dundee United when I was covering the game and he was good.
“He went down to Hull and his career fizzled out there but Liverpool took him for £10m and now he is on fire.”