Jurgen Klopp was understandably frustrated watching his Liverpool side being battered by Manchester City at the Etihad on Saturday.
The Reds went a goal behind when Sergio Aguero found the back of the net early in the first half before Sadio Mane was then shown a straight red card.
The Senegalese caught Citizens keeper Ederson in the face with a high foot and was given his marching orders by referee Jon Moss - a decision that changed the game immeasurably.
From there City scored goals for fun with Gabriel Jesus and Leroy Sane both bagging doubles for Pep Guardiola's rampant home side.
Klopp appeared furious watching Liverpool's capitulation, and explained exactly why he was "really angry" after the Premier League fixture.
"We have a lot of things to learn from this game," Klopp told liverpoolfc.com.
"I was really angry at half-time. Not angry because of the result but because the boys saw we could have won at 100 per cent.
"It's not important any more, we lost 5-0, it sounds crazy I know! The first half, we didn't play like we wanted to, we were 60-70 per cent."
Many of Klopp's players had been away on international duty ahead of the game, and the German suggested this may have affected their ability to hit top gear.
"Sometimes it's impossible after the international break - different plans, different positions, they played all over the world, I don't like this but we cannot change it - but you saw City struggled also a little bit," he added.
"We had then these clear moments and then it's one goal conceded, forget it, it can happen, but the red card gave them the advantage.
"Everybody knows it's a bad place to be one man down and we could see this so we tried to drop a little bit deeper, but it makes completely no sense.
"We have a lot of things to learn from today, but the problem is today we cannot change it, we can only learn from it and make the next time better."