Chelsea were stunned by a 1-0 loss to Dinamo Zagreb in their opening Champions League game on Tuesday.
The Blues lost courtesy of a 13th minute goal from Mislav Orsic at Stadion Maksimir.
Thomas Tuchel's side were well off their best in the Croatian capital.
Dinamo came close to a second but Stefan Ristovski's thunderous, long-range effort was superbly tipped on to the bar by Kepa Arrizabalaga.
Pierre-Emerick Aubameyang was handed his Chelsea debut and largely looked off the pace.
Reece James rattled the post as Chelsea launched a fruitless late onslaught.
"[I'm angry at myself] because I didn't see it coming," Tuchel said in Croatia on Tuesday night, via the Evening Standard. "Obviously I was in the wrong movie. I did not see that coming.
"I thought that the last game helped us, I thought we showed a reaction in a very difficult moment against West Ham, got the reaction, got the result in the moments when you need a bit of luck in the situation we were in.
"We had good training sessions, I thought the team was prepared, I thought we know what this is all about. I didn't see it coming, that's why I'm angry on myself."
Tuchel added: "I'm angry with myself and I'm angry with our performance. This is a huge underperformance from all of us. It's not precise enough, it's not clinical enough, it's not aggressive enough. It's not determined enough and not enough individually and not enough as a team and that's why we lose this game
"It's for me to find the solution and find the reason why. Obviously they are underperforming individually and I don't really know where this performance today comes from.
"A lack of determination, a lack of hunger and a lack of intensity to actually do the things that we need at the highest level. We are clearly not where we want to be
"We had an amount of deliveries and an amount of spaces and an amount of ball wins that should be by far enough to have far more attempts but we did not. We conceded a goal with two players in a counter attack [which] never happened before.
"I don't understand why it happened today. It's clearly my job to analyse it so this is my analysing from the sideline and it will continue. We need to be much better. We are not finished, we are not happy with our results, we are not happy with the way we play in general, but I thought we were in a good way.
"I'm a bit surprised by this performance today. There is no day off. There is no day off in the planning and there cannot be a day off in the moment."
