After Thursday’s first-leg loss, Greece boss Jovanovic made six changes to his starting line-up, including a start for Karetsas.
With an average age of 23 years and four days, it was the second-youngest team named in this edition of the Nations League, and the youthful talent came to the fore.
PAOK winger Konstantelias, 22, had a hand in all three goals, setting up the second and third after smashing the first beyond Craig Gordon.
But it is Karetsas who will take the headlines. He scored the pick of the goals and looked so at ease in the international arena.
“For 17 and what he’s doing, the goal tells you… He isn’t rushing it and blasting it,” Naismith said. “He’s got the composure.
“As a coach, these are the types of players who are really hard to find, willing to be aggressive and go forward. His first thought is to play forward and beat his man rather than be safe.
“He influenced the game on Thursday and tonight, that’s the sign of a very good player.”
The Genk attacking midfielder, who played off the right at Hampden, only made his club debut at the end of last season and has established himself as a regular in Belgium this season before making his international bow in the first leg.
“He’s been class, picking up really good positions, comfortable in possession, and made great decisions as well,” McFadden said when the 17-year-old was replaced in the second half.
“It was an incredible finish – we’re going to be seeing a lot of this kid going forward.”
