Chelsea are three points worse off than they were this time last season and slipped below Arsenal in the table on goal difference - Charlie Daniels' 95th-minute header putting the cherry on that particular cake.
Wednesday marked Sarri's third inquest in his short reign as boss, after doing so following losses to both Tottenham and Arsenal in the league.
He left the media waiting for over an hour but eventually conducted all his contractually obliged post-game duties, as he detailed what he spoke about with his players.
“In that moment I wanted to talk with them alone, without anybody else. As I said before, I needed to understand. It's impossible to play in such a different way between the first and second half. I don't understand why," Sarri said.
"I want to try again tomorrow because we want to solve the problem," the Italian continued. "We have only to say sorry to our fans. We can lose, of course, in every match, but not in this way. We need to solve the problem. We need to understand where the problem is.
“It's very difficult to understand the performance. We played very well in the first half, but in the second half we conceded the goal after two minutes and stopped playing. In the offensive phase, we played not as a team, but as individual players, and did not defend.
"It's hard to understand why. From what I saw in the first half, there was a possibility to solve the problem. We had 43 minutes still to play, so we could solve the problem. But not with that kind of reaction. So it's very difficult for me to understand why.
"I wanted to try to understand after the match. I spoke with the players immediately after the match, but it's very difficult also for them to say why. So I have to try again tomorrow because I need to understand. I need to solve this problem.”
Chelsea fans chanted "you don't know what you're doing" after Sarri subbed Gonzalo Higuain in the 65th minute for Olivier Giroud on their new recruit's first Premier League start.
Bournemouth's win comes almost a year to the day since they beat Chelsea 3-0 at Stamford Bridge on January 31 last season. The Blues also lost by a four-goal margin for only the second time in the club's Premier League history, last doing so in 1996 after losing 5-1 to Liverpool.
Chelsea were defensively weak but have also only scored 13 goals in their last 13 games, with Higuain not yet making an impact in his first two matches.
Sarri says he doesn't feel under pressure over his Chelsea future, but worries that he is failing to reach his players.
"I feel frustrated, not under pressure. I didn't see any signs of my work, so I am frustrated,” he added. “Maybe it's my fault, maybe I'm not able to motivate them."