The German keeper was ejected in the 18th minute for towing down Samuel Eto'o at the edge of the area.
"The goalkeeper was going out of the penalty area and was taking down the attacker from Barca, taking him away from a goalscoring opportunity. That is a red card situation," Hague told Sky Sports on Thursday.
"I was close to the situation, I whistled and after the match the red card was correct. I don't regret it. I had to take the decision from my position. You can discuss the situation but cannot regret it as referee of the day," he added.
Hague admitted he could have awarded Barcelona a goal if he did not act too quickly in his decision to give Lehmann his marching orders.
"Three to five seconds later the ball was going into the net and if we had waited three, four or five seconds more we could have accepted the goal to Barcelona," he said.
He also refused Arsenal coach Arsene Wenger's statement that the Catalan's equalizer should have not been counted as it was offside.
"I saw play going very fast from my position and cannot see any offside," the Norwegian said.
"I have an assistant who takes that decision. He was in line, and if I'm not sure the benefit of the doubt goes to the attacker," he added.