Barca and Borussia Dortmund finally agreed a fee of 105 million euros (96.86 million pounds) plus bonuses on Friday after being locked in negotiations for weeks, during which time the player was suspended by his club after failing to report for training.
The France international, 20, is Barca's second signing since the departure of Neymar for a world-record fee of 222 million euros in August, with Brazil midfielder Paulinho also joining.
Valverde believes Dembele will restore sharpness to his side's attack.
"He's a player who guarantees an attacking threat, which is something we had lost and we badly need," Valverde told a news conference ahead of Barca's game at Alaves on Saturday.
"He can play on either wing or through the middle, he gets forward a lot and is very technical. We expect a lot from him and we're very excited about him."
Dembele, 20, has been playing top-flight football for less than two full seasons but is Barca's record signing, his fee far outstripping the 81 million euros the club paid Liverpool for Luis Suarez in 2014.
He also replaces Gareth Bale as the most expensive player in La Liga and, in price terms, is second only to Neymar.
"We know what things are like in this market as the deadline approaches and everything becomes disproportionate," Valverde said of the money paid for Dembele.
"It seems like every club is going to sign the most expensive player of all time now because the prices are so high after Neymar's transfer. But when a player is on the pitch they don't worry about how much they cost."
Valverde, whose side won their opening game of the Liga campaign 2-0 at Real Betis last week, highlighted Dembele's astonishing rise from the reserve team of Stade Rennes in France to Barca in under two years.
"His career is amazing. He was barely known only a few years ago but he's a real quality player," he added.
"Barca have been interested in him for a while along with a couple of other teams. We have invested a lot of hope in him and we hope he can give us a lot of happy moments."
(Reporting by Richard Martin)
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