There are instances that call for experience but there are times when you give youth a chance to develop. Dembele needs to move if he's going to develop as a player. The natural ability is there but things like decision making that come with game time won't happen under Valverde.
He's safe and cautious in his approach to management and if Valverde had control over all the transfers, not a chance a player like Dembele would be anywhere near his radar.
I don't see Valverde being brave enough to throw him in ever when there's any more than a 10% risk in the game at stake. He always reverts to experience, it's just what's natural to him.