Dembele became the second-most expensive footballer in the world when he moved from Borussia Dortmund in the latter stages of the summer transfer window, signing a five-year contract with the Spanish giants.
"I remember two games especially," Dembele told Sport. "One against Chelsea, when Iniesta scored the goal to qualify for the Champions League final. When I see Andres now I remember it. I can't wait to tell him.
"And the other was a Champions League game against Arsenal. Barcelona had not done great in the first leg but in the second leg they produced a brilliant performance, with four goals from Messi. That's when I realised Messi's not from this world. It was incredible.
"I had a poster of Iniesta. He seemed like a different player to me: elegant, special... a genius. I watched all the Barcelona games to see how they played and the geniuses, Iniesta and Messi. After, I tried to imitate them but it was difficult. They do things that no one does. Messi always plays at a sublime level. He's the best player in history!"
Dembele is expected to make his Barcelona debut in next weekend's La Liga clash with Espanyol.