Sancho will face Spurs in their Champions League last-16 first leg clash at Wembley on Wednesday.
The north London attempted to buy Sancho before he joined Dortmund from Manchester City in 2017.
Asked about the huge impact Sancho has made, Pochettino said: "He doesn't surprise me.
"We knew very well that he was capable of doing what he's doing now, with game time.
"The quality he was showing at Manchester City and with his national team (at age group level) - we were talking in the last few years, that he was a massive prospect to be a very good player.
"He is showing now in Germany that he can cope with the pressure of playing in the Bundesliga and Champions League.
"And it doesn't surprise me, his talent is there."