Maurizio Sarri was mystified by Chelsea's sudden drop-off at Everton and admitted a Premier League top-four finish would be "impossible" if they do not improve.
The Blues dominated possession and territory in the opening half at Goodison Park, and Eden Hazard saw a deflected strike come back off a post, but the visitors faded badly after the interval.
In contrast, Everton were rejuvenated and sealed a 2-0 win by virtue of Richarlison's header shortly after the resumption and Gylfi Sigurdsson scoring on the rebound after his 72nd-minute penalty was saved.
The result keeps Chelsea three points behind fourth-placed Arsenal and, though Sarri was enthused by his side's first-half display, he lamented their fragile mentality thereafter.
However, the Chelsea head coach refused to write off their hopes of finishing in the Champions League places, saying: "It's not impossible, why impossible?
"We need to recover only three points so it's not impossible in eight matches. It's impossible if we are not able to improve, if we are not able to avoid a second half like that.
"We need only to play like in the first half for the next eight matches. We played probably the best 45 minutes of the season then suddenly, at the beginning of the second half, we stopped playing. I don't know why.
"The situation is clear, it's impossible that the problem is a physical problem because if you have a physical problem, you can go down gradually, not in one second.
"We continued really very well until the last minute of the first half and (were playing) suddenly very badly in the first minute of the second half.
"It's very difficult for the players to explain to me the change and it's very difficult for me to explain to you the change. But probably it's a mental block, I think.
"I am worried about our mentality but we played the best 45 minutes of the season."
For Everton it was a first win against a top-six side since January 2017, when they demolished Manchester City 4-0 on Merseyside.
Manager Marco Silva has stressed that is the sort of run which has to be eradicated if they are to fulfil their ambition of challenging that established Premier League elite.
"I didn't know these numbers before the match," he said.
"It is something which cannot happen again because it is not a normal thing for a club like us, so many times not winning against these strong sides.
"It is not an easy thing but it is something we should be able to do more times - even this season we have had opportunities to to this."
After such a disappointing opening 45 minutes, Silva said he knew he had to rally the players at half-time.
"It was too clear the difference," he said. "I said to our players something simple: 'We have to do something different if we want to win the game'."