Adama is being celebrated in Spain for his impressive debut in this week's 0-0 draw with Portugal.
Karanka told Marca: "Victor Orta (sports director) and I knew Adama from his time at Barcelona, but his signing for Middlesbrough seemed financially unviable. When we proposed to change (Albert) Adomah for Adama they told me if I was crazy.
"Adomah was a proven winger who had been instrumental in the promotion and Adama was a 'kid'. He came from not playing just for Aston Villa and had had some injuries. I took responsibility because I believed he was worth it and time has shown that I was not so misguided.
"Adama went from being at Barça to seeing himself alone at Aston Villa and suffering a decline. He had a bad time. Coming to the 'Boro' with a Spanish technical 'staff' and being surrounded by Negredo, Barragán, Valdés ... helped him a lot.
"He also began that marvelous physical change that he has undergone and that is obvious. I remember that as soon as I arrived, I organised a meeting between his personal coach and the club coach. I was clear: 'We have a diamond on our hands and we have to polish it. I don't want a single contradiction'.
"That boy had quality, speed, power ... He had it all! Our challenge was to improve his positioning and his collective game. At Barça everyone was looking for him because only with his physique he won matches. In the 'Boro' it was vital to put his potential at the service of the team.
"Adama understood it perfectly and we shared many hours of video analysis. He learned fast."