The pair had a running battle during the 1-1 draw, with Sport reporting Sergio called Umtiti a "black s***" while in action.
At the final whistle, as the players made their way to the tunnel, Sergio is alleged to have repeated the insult. A source says "Umtiti went crazy" and made a beeline for the Catalan "wanting to attack him".
But Umtiti was held back by Barca teammates as they ushered him forcefully into the away dressing room.
Ref Jesus Gil Manzano noted the melee in his match report, but did not include any notes on the manner of the insults nor any physical altercations.