The goalkeeper, who was this week named runner-up to Gianluigi Donnarumma as Europe’s finest No1, was also culpable when conceding a penalty in the 3-2 defeat.
Mendy twice had the chance to clear the danger after Jorginho’s under-hit back-pass, but was panicked into bringing down Jarrod Bowen under pressure from the winger.
He was then beaten by Arthur Masuaku’s swerving cross late on, which secured the win for the home side.
Tuchel said: “Edou is dealing with a rough period. I don’t know why exactly, but he seems to have lost a bit of confidence. Okay, his decision-making was not on the highest level today with the first goal, but we also bring him to a really awkward situation.
“He could have saved the situation, but he did not. We are far, far away from blaming anybody, but we need to admit that we do too many crucial mistakes at the moment that cost us a win against Man United and today as well.”
Chelsea’s manager was heavily critical of his players after their 2-1 win against Watford in midweek but despite their sloppiness costing them on Saturday, he refused to lay into the team after only their second Premier League defeat of the season.
He added: “I didn’t see our performance dip against Man United or today. I think the performance is enough to even win the game, but what we clearly see is we give easy goals away and this is very unusual.
“We make big mistakes that lead to big chances that lead to easy goals. We do this now in three matchdays against Man United, Watford and again today. If you do this at this kind of level, you can’t expect to have a result.
“The (title) race is not interesting at the moment. What is interesting is always the next 90 minutes and what is interesting is to have the focus right, the attitude right, and to take care about the details.
“We are not good enough on the details at the moment. We do too many big mistakes and have to care about the details.
“There is no need to ask questions or start doubting about the big picture. This is a game if you play at West Ham about the details. I can only repeat myself: without doing big mistakes, there is a big chance we win this game.”