With Messi ruled out of Barca's Champions League tie against Inter, Dembélé was surprisingly left out for Rafinha.
The France international arrived from Borussia Dortmund as the replacement for Neymar in a deal that could rise as high as €145m and the absence of the club's attacking talisman was seemingly the ideal time to show his worth.
But he let the opportunity slip away from him due to the sloppiest of mistakes: bad timekeeping.
According to Sports Cuarto, Dembélé arrived 25 minutes late to a pre-match meeting ahead of the Inter Milan game at the Camp Nou.
Manager Ernesto Valverde had called for a team meeting to be held at 7pm, two hours before kick-off, but Dembélé did not show up until almost half past.
It's not clear whether Valverde intended to use him in the game, but he watched the entire game from the bench as the Blaugrana secured a 2-0 victory at the Camp Nou.
Rafinha Alcantara - brother of former Barça midfielder Thiago - started in his place and made the most of his chance by scoring the opening goal, volleying a Luis Suárez cross home with style in the 32nd minute.
This unfortunate episode will not do Dembélé's chances of playing in this weekend's Clásico any favours and could see him fall behind summer signing Malcom in the pecking order.