Tuchel worked with Dembele at Borussia Dortmund.
He said, "Ousmane is an incredible player, very, very talented. I could see in each training that he had an extraordinary talent. But he's also an extraordinary boy, very nice, but a little crazy.
"With him, it was a lot of laughs. We never got bored. If he focuses, he can achieve all the goals he wants. When I spoke to him for the first time, he told me he wanted to play for Barcelona.
"He is there today, which proves that he can succeed in what he has in mind. But he must stay focused, and improve again."