Marco Silva could not understand why Everton turned in a "nervous" performance as they lost 1-0 at home to Leicester City on New Year's Day.
Everton have now won just one of their last eight games after Jamie Vardy struck a second-half winner at Goodison Park following Michael Keane's error, but Silva offered no excuse for his side's display.
Bemoaning the nature of the defeat, the Everton boss promised to look into the reasons for a scrappy performance in which they made mistakes at the back and lacked fluency in attack.
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"It's a disappointing result and a poor performance, to be honest," he told Sky Sports.