The Frenchman will not play against Olympique Lyonnais, as he is yet to recover completely from a muscle injury.
Barcelona is set to host Lyon at Camp Nou stadium on Wednesday. Dembele's participation is doubtful after he sustained a slight hamstring strain in his left leg during his side's 3-1 La Liga win on Saturday over Rayo Vallecano.
"If he will not be fit then he will not be included in the list," Valverde told the press at the club facility in the Catalan capital.
"Yes, it is a decisive match, but we cannot take a chance to lose a substitution," the coach added.
Valverde said there is no favourite for Wednesday's clash in fact it might go to extra time if the first leg's 0-0 draw is any indicator to the second leg.
"(Lyon) has fast and powerful players," he said, adding: "It is a squad used to taking initiative in the league (France's Ligue 1)...
"Lyon's play is a thing that cannot be changed from one day to the next. I imagine that they realize the value of scoring an away goal and that they will give it a try.
"This means that there will be surprises... They are a great team and will focus on winning, just like we will," Valverde added.
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