Dembele's spectacular winner with 12 minutes remaining sealed a 2-1 victory for Barcelona in Tangier on Sunday.
"I don't know if he needs matches like today, that is a question for him," head coach Valverde said when asked if Sunday would have boosted Dembele's confidence.
"He is with us and we have high expectations for him. He has great quality and is here with us.
"He took part and he scored a goal, which I think is something to be happy about. As the game went on, he got better."
Sevilla hit the front thanks to Pablo Sarabia inside nine minutes but Barca equalised three minutes prior to half-time via Gerard Pique.
Dembele then put Barca ahead before Marc-Andre ter Stegen saved Wissam Ben Yedder's last-gasp penalty.
Valverde added: "We are happy because we were competing for a title against a great opponent.
"We knew that the circumstances of this match were different. It was a very difficult match because we started off by falling behind and we had to mount a comeback.
"Many players came into this game after time off, having only just returned this week, and this was obvious.
"We got better as the game went on. It was a good match, in which they also had chances.
"It is good to win because it boosts you in what you're doing and allows you to start the season in a positive way."