The Gunners, who have endured a miserable start to the season, went down 2-1 to their north London rivals to leave them on just seven points after as many games.
Head in hands: Wenger oversaw Arsenal's fourth defeat of the campaign
Rafael van der Vaart opened the scoring for the home side shortly before the interval only for Aaron Ramsey to draw Arsenal level on 51 minutes.
But it was Tottenham who had the final say when Kyle Walker drilled the winner past a flapping Wojciech Szczesny 17 minutes from time.
Bad day at the office: Szczesny flapped at Walker's second-half effort
'Tottenham's second goal made the difference,' said Wenger, who faces three months without Bacary Sagna after the defender broke his leg in the derby defeat.
'After that they could sit back and play on the counter-attack. That is why we are guilty because we did not do our job properly, we didn't defend properly on the winning goal. It is two steps forward and one step back.
Down and out: Sagna faces three months out of action after breaking his leg
'Sunday was a game we could have come out of with a point but it [our inconsistent form] is down as well to a lack of confidence.'
Arsenal are without a win in their last eight away league games, which is their worst run in the Premier League.
But Wenger insists his injury-hit side will bounce back, despite their disappointing start to the campaign.
All smiles: Ramsey's joy was short-lived after Walker scored the winning goal
'We will come back because you could see again that there is a strong spirit in the team,' the Arsenal boss added.
'But we have to become a bit more realistic and do our job better in certain situations.
'I felt on Sunday that defensively we are becoming stronger. But we lost the ball from good positions in the final third so we could have done better with our passing.'
Redknapp 'disgust' at sick chants as Allen brands Wenger 'two-bob'More woe for Arsenal as Sagna is ruled out for three months with broken leg Tottenham 2 Arsenal 1: Walker strikes to settle north London derby and pile misery on Wenger