Dembele was not part of Valverde's squad for Saturday's shock 3-1 loss in LaLiga, despite being available again after serving a two-game suspension.
Barca let slip a one-goal lead as they conceded three times in the space of seven minutes in the second half, their hopes of a recovery not aided by the loss of Luis Suarez to a calf injury before the interval.
The presence of Dembele on the bench may have helped Valverde at the weekend, though the Blaugrana boss is adamant the France international must show he deserves to be regularly involved.
"Everyone always has to earn their place - Dembele is no exception," Valverde told the media ahead of Tuesday's Champions League game against Slavia Prague.
"When he plays, he can stay in that position [if he plays well], as happens to Ansu [Fati], Luis [Suarez], [Antoine] Griezmann or [Gerard] Pique.
"When you have your chance, you will have to try to stay [in the team]."
Valverde was also questioned over the future of Riqui Puig amid reports the midfielder may push to leave in January to get first-team football – he is yet to feature for Barca so far this season.
"I don't know what can happen in the next transfer window - there is still time," Valverde said when asked about the rumors the 20-year-old is considering his future at Camp Nou.
"We have a lot of midfielders. With his talent, it is clear he will play at Barca. What we do not know is when."