Rangers manager Walter Smith rated the 3-0 defeat by Celtic as his worst Old Firm experience since his return to the Ibrox helm in January 2007.
"We are better than what we showed but give credit to Celtic," said Smith.
"They capitalised on our errors and deserved to win. We never really started the game at all. Our defending was poor, our possession was poor.
"All in all, it was as poor a performance as we have had against Celtic since I came back."
The Scottish champions slipped eight points behind Celtic in the Scottish Premier League as Gary Hooper netted a first-half double and Kris Commons added a long-range strike.
Rangers have two games in hand on their old rivals but Smith concedes his side will have to improve markedly to close the gap.
The visitors were toothless in attack during the first half and only managed a slight improvement after the interval when they were chasing the game.
"I don't think you can say our team lacks belief or character because our team has been playing well over the last few years so I don't think it was anything to do with that," continued Smith.
"It was a poor performance from our point of view and we are disappointed about that. We didn't look as though we had the necessary edge and that was a worrying factor for us."
As far as Smith is concerned, Rangers now need to lift themselves for the remainder of the campaign, having not quite hit top form so far in 2011.
"Everybody looks at the Old Firm games but I don't think we have been playing that well since the turn of the year," he added.
"We've been patchy with our form although we have managed to win some of the games so we will have to look at getting a better level of consistency about us.
"If we can gain that consistency, we always have the final Old Firm game of the season at Ibrox, which will take on importance if we can take on a level of consistency. We are going to have to search for something that will give us a lift in the remaining part of the season."
Steven Naismith pulled up with an ongoing hamstring problem late in the first half and is now facing several weeks on the sidelines.
Smith said: "He's had the injury for a few weeks now and I dare say now he will have to be out for a few weeks to clarify and rectify exactly what's wrong."
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