Mason Mount pointed to "tired minds, tired bodies" as he assessed the mistakes that cost Chelsea in their 3-2 Premier League defeat at West Ham.
Jorginho's poor back-pass played Edouard Mendy into trouble, leading to the goalkeeper conceding a penalty which Manuel Lanzini converted, before the shot-stopper made another error late on, allowing Arthur Masuaku's cross to sneak inside the near post.
Mount, who scored a superb volley to make it 2-1 to the Blues shortly before half-time, hinted the fact Chelsea have already played 23 fixtures across all competitions is starting to catch up with them.
"We have had a difficult week and the game midweek [a 2-1 win against Watford] we came away with three points where maybe we shouldn't have," Mount told the BBC.
"We made mistakes, the same today. We are making mistakes we haven't done for a while and need to bounce back now.
"A lot of games, it can take a toll, but that's not an excuse we can use – we come out here and want to win, it's a derby. We don't concede many, so that is something we need to look at."
He suggested the errors were only recently creeping in, and added: "Tired minds, tired bodies, but we need to keep the standard high. We regroup and come back next game."
48 - Chelsea’s loss to West Ham was their first Premier League defeat after leading at half-time since December 2018 against Wolves under Maurizio Sarri – they had been unbeaten in 48 league games when ahead at HT before today (W40 D8). Slip. pic.twitter.com/uQFP4Ix4gU— OptaJoe (@OptaJoe) December 4, 2021
Chelsea head coach Thomas Tuchel also bemoaned individual errors but felt his side's display was still good enough to have earned all three points.
"I don't think we did a bad match, it was okay, we can win with this performance," Tuchel told BT Sport.
"It is tough to play here, we did too many individual mistakes, we did already against Manchester United and Watford and got punished. If you want a result at this level you have to minimise mistakes. We talked about it before, it's obviously not helped."
Looking at the mistakes that led to conceding the penalty, Tuchel said: "Everybody is involved, the pass is not the best decision, the orientation is not the best. We can save it, but the decision-making is also not the best from Edou in this moment.
"We created a lot of half-chances, a lot of touches in the box, but sometimes you need a bit of luck, a deflected shot or loose ball that falls to our feet. Their third one is a strange one, the two big chances they had in the first half were our mistakes.
"We could not finish it with clear and better chances, be more ruthless; we struggled also in defending some situations.
"Every loss is a bump in the road, that will not hold us back from what we demand from us, but we need to play more accurate, more stable and reduce big mistakes."
The scoreline marks just the third time Chelsea have conceded more than once in a game in 53 outings under Tuchel and was the first such instance this season, with the Blues having shipped just six goals in the 14 league outings prior to the defeat by the Hammers.