El inexplicable cambio de Valverde

Ousmane Dembele was, for many Barca fans, the best player in Ernesto Valverde’s team when the coach decided to take him off after 67 minutes in Lyon. Few understood the change with Luis Suarez misfiring and Leo Messi well martialed, as the Frenchman was therefore the most dangerous part of the attack. Why Dembele?