Gerard Pique believes referee Alejandro Jose Hernandez Hernandez's decision to dismiss Real Madrid defender Sergio Ramos in the 83rd minute of Saturday's Clasico effectively cost the Catalans the game as they lost control after the centre-back's red card.
Ramos received his second booking of the evening after a foul on Luis Suarez with the score at 1-1 seven minutes before the final whistle, but Cristiano Ronaldo then somewhat surprisingly earned Madrid all three points with a goal just two minutes after the Spain international's exit.
"We did not know how to play with a player more on the pitch and we lost control after the red card," Pique told Barcelona's official Twitter account.
Saturday's defeat leaves Barcelona just six points clear of Atletico Madrid in the Liga table, while Real Madrid are also back in the title race, now seven points off the pace.
Nevertheless, Pique sees no reason to panic as the Camp Nou side's 39-game unbeaten run in all competitions came to an end, stressing they are still in good shape to win silverware this campaign.
"We have to move on and not dwell on this defeat. You cannot always win.
"We are still in a good position in three competitions."
Barcelona will be looking to return to winning ways on Tuesday, when they host Atletico in the first leg of their Champions League quarter-final tie.