The La Liga champions confirmed Dembele’s arrival from Borussia Dortmund on Friday, in a transfer which could rise to €147million.
It almost doubles the €80m they paid Liverpool for Luis Suarez in 2014, the club’s previous record purchase.
Dembele, 20, will arrive in Barcelona over the weekend and will sign a five-year deal at Camp Nou on Monday.
“Dembele gives us width, something we lost and we needed,” Valverde said in a news conference on Friday.
“He can play on the left or the right, or even through the middle, he is quick and he is technically good. We hope he’s going to add a lot to the team. We’re really excited to have him.
“[His rise] has been impressive. A couple of years ago he was hardly known by anyone, but he’s a player with a lot of quality. Barca have been following him for a while, like other teams, and last year [with Dortmund] he proved he’s a top-level player. We have a lot of hope deposited in him.”