Harry Redknapp was at a loss to explain what had gone wrong for Tottenham following their 5-2 loss to Arsenal in the North London derby.
Spurs looked set to keep alive their Premier League title hopes after goals from Louis Saha and Emmanuel Adebayor opened up a commanding lead inside 35 minutes.
However, Arsenal always looked threatening and managed to get back on level terms before half-time, when Redknapp tried to regroup.
He sent on Sandro and Rafael van der Vaart for Niko Kranjcar and Saha, but the momentum was with the Gunners and Tottenham had no answer.
Such an emphatic defeat has become a rare event under Redknapp and he is not sure why they performed so poorly, but recognises that it is important to respond well ahead of next weekend's meeting with Manchester United.
"Even at 2-0 I couldn't say I felt over-comfortable," the manager told Sky Sports.
"I wanted to get to half-time (at 2-0) but I felt they were creating chances and we looked a bit open really. It was a difficult one. I felt at half-time I needed to try to thicken us up a little bit.
"That should have happened, but didn't. We didn't defend well. It was basic, really."
Redknapp added: "We don't get beaten like that too often. It is a difficult one to put your finger on really.
"Whether the occasion got to one or two of them (I don't know), but we never really got hold of the ball."