Chelsea boss Thomas Tuchel believes his side's mistakes in recent games have been more technical rather than structural.
The Blues have only won one of their last three matches with a draw against Manchester United, a victory at Watford and a defeat away at West Ham.
Mistakes in their previous game at the Hammers in particular saw Chelsea lose all three points despite leading the match twice.
Speaking to the media ahead of his side's Champions League tie against Zenit on Wednesday evening, Tuchel gave his thoughts as to why his team have made mistakes in their previous few games.
"We have to constantly be on our highest level. Teams try to press us high. We had games, many games where teams tried and we found good solutions.
"It is the mistake we did now against Watford and West Ham is in a high pressing moment but it is still like, if you see Jorginho doing a mistake like this, we do not have a structural or behavioural problem.
"It is more a technical or decision making problem. This refers to mental or fatigue."
Errors from Edouard Mendy and Jorginho allowed West Ham to get themselves back into the game on Saturday, with the Irons winning the match in the latter stages.
Chelsea's loss was only their third of the season in all competitions and only their second in the Premier League.
They now enter a busy festive period of fixtures, with six more games to play after Wednesday before the New Year.