Rangers manager Walter Smith admits the 3-0 defeat to Celtic was his worst Old Firm experience since his return to the Ibrox helm in January 2007.
The Scottish champions capitulated at Parkhead, as Gary Hooper claimed a double and Kris Commons netted for a second time against the Ibrox side since his arrival in January.
The win allowed the Hoops to move eight points clear the top of the Clydesdale Bank Premier League, albeit Rangers have two games in hand, and Smith concedes his side simply were not good enough.
"We are better than what we showed but give credit to Celtic as well," said Smith. "They capitalised on our errors to get in front and overall deserved to win. We never really started the game at all. Our defending was poor and our possession was poor.
"All in all, it was as poor a performance as we have had against Celtic since I came back. We deserved no better than we got.
"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. It was a poor performance 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."