Chelsea manager Frank Lampard believes his side are suffering from fatigue after they lost 2-1 at Wolves on Tuesday.
The Blues took the lead at Molineux through an Olivier Giroud goal, but Daniel Podence scored to pull the hosts level before Pedro Neto netted the winner deep in injury time.
The loss is Chelsea's second in a row, having gone down 1-0 at Everton on Saturday.
Lampard admitted his side may have been lacking some energy, but insisted that is no excuse for the defeat.
"There could be some fatigue. I hate to say it after a defeat because it could sound like an excuse but even the first 60 minutes when we were the better team we didn't have a zip about us," he said in a press conference after the match.
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"We have travelled up to Everton, come back and up to Wolves but everyone is in the same boat on that front. At the moment we have a couple of injuries in areas that mean I can't make too many changes or rotations in certain areas of the team.
"So I understand that side of it but when you are 1-0 up and turn the result around I wouldn't say it was because of fatigue."
The former midfielder was asked if injured duo Callum Hudson-Odoi and Hakim Ziyech are nearing a return to the team.
"Hakim and Callum will hopefully be training by the end of the week," he said. "Whether they are fit or not for West Ham, I'm not sure."
Lampard said his side were caught out by threat Wolves posed in counter attacks, but took some positives from their performance despite the loss.
"We didn’t play enough. It is pretty simple. The threat from Wolves is clearly the counter attack and it is a major threat from what they have got," he said. "It is the main one because they have speed and quality in forward areas. The players knew it before the game, the players knew it in game but we allowed some counter-attacks.
"If you are going to allow a team to play to their strengths then you may lose that game.
"The real positive things we've seen is that we're becoming more secure. We've been given a different kind of game in these last two games, particularly Everton making it difficult for us to break them down. We didn't really succeed in that.
"Wolves play very much with a counter attacking style and we didn't deal with that. The players have to feel that on the pitch. The message was clear: they counterattack. That's disappointing."