Roberto Martinez conceded Everton turned in their worst performance of the season in the 2-0 defeat against Arsenal at Goodison Park on Saturday.
The Toffees offered little in slumping to a ninth Premier League loss of the campaign as goals from Danny Welbeck and Alex Iwobi moved Arsenal within three points of second-placed Tottenham.
Martinez was disappointed his side could not replicate their stirring 2-0 defeat of Chelsea in the FA Cup last Saturday, telling BT Sport: "We have not performed. We were not clear in what we were going to do, we were second best.
"Two teams... one played on Wednesday [Arsenal in the Champions League], and we looked like it was us that had had the extra game. It was so disappointing, a contrast to what we did seven days ago.
"We had second thoughts on every action and Arsenal were better than us in every aspect.
"We missed Gareth Barry but that's not an excuse."
Martinez was at a loss to explain Everton's inconsistency, which has left them drifting in mid-table.
He added: "It's not a lack of quality, the team we have now has a really good mix of youth and experience and can cope with the expectations. We have no-one to blame but ourselves.
"I haven't seen us play as badly this season. We had a bad 45 minutes against Manchester United but, apart from that, we have always been ourselves. [This] was very different.
"You have to compete and have the intensity we had seven days ago, that was the perfect example of what we should be. But the performance was not good enough and nowhere near our usual standard."