Talisman Lionel Messi's 23rd-minute goal appeared to have Barca on track towards a fifth straight home win and three points that would have seen them leapfrog rivals Real Madrid and table-toppers Atletico Madrid in LaLiga.
However, Darwin Machis leveled and Koeman was sent to the stands prior to Jorge Molina heading home the winner 11 minutes from time to leave Barca two points off the pace with five games to play.
Koeman insisted he did not say anything insulting to the official and hinted he may challenge the decision.
"I don't understand why they have sent me off. They [said I] have disrespected the fourth official," he said in quotes reported by Marca.
"I don't understand the red. According to the delegate, it was a lack of respect for the fourth official.
"I haven't said anything. I haven't insulted him or anything. But hey, if the fourth official wants to be the star of the night ... thank you.
"Let's see what the record says. If the record says something that I haven't said, I'll have to do something."
Koeman accepted his team made errors in a defeat few would have seen coming but the Dutchman is refusing to lose belief in a thrilling title tussle.
"We lost focus in the second half and made mistakes in defense," he added.
"We must accept today's defeat. It is reality. We had opportunities to win, but okay we have five games to go. Each one will have its difficulties. We have to prepare well for next Sunday.
"There are still possibilities. We have to accept defeat, analyze it and prepare for this tournament.
"I think we have tried to continue. We have not created many opportunities after their goals. They have closed down well and we could not create more.
"Seeing the two goals, we leave spaces where we have to close them. We have not been well defensively.
"It is a hard blow to stop depending on us. We are disappointed. We have five games to be champions. For us it is the same as for others."