Craig Levein claims Charlie Mulgrew is simply the latest in a list of late developers to play their way into the Scotland team.
The Scotland boss believes Mulgrew, who will play at left-back against Slovenia, is just like several players in the past who have emerged onto the international scene after the "penny has dropped".
Levein told Press Association Sport: "I have had Paul Hartley and Barry Robson, to name two, who have taken their time but the penny has dropped and they have kicked on and the best part of their careers have been the latter part. I really take to them because they are trying to make up for lost time and Charlie is another one."
The Celtic defender featured 11 times for Scotland under-21s but makes his first appearance for the senior team in the friendly against Slovenia in Koper at the ripe old age of 25, as Levein continues preparations for the World Cup qualifiers which begin in September.
Levein added: "The penny has dropped in his mid-20s and he has improved enormously. So I am delighted for Charlie, it is a great honour for him.
"Everybody can improve at any age. It is a mental thing, a career thing, it is a decision that players make to knuckle down, work hard, and do their best every single week.
"It is hard to say what happens exactly but I have seen it happen on a regular basis.
"He would have been in my squad for the last game against Cyprus in November but he had an injury. I spoke to him and he was gutted.
"If you had said me to me two years ago that Charlie Mulgrew would be a starter for you at international football I would have said no, because in my mind, at that time, he wasn't.
"But he's just playing out of his skin. So credit to Neil Lennon and Celtic for the faith in him and for the faith Charlie has in himself."