The Dutchman praised his side’s determination, aggressiveness, and clinical finishing as he watched his side inflict Pep Guardiola’s worst ever defeat as manager.
Goals from Romelu Lukaku, Kevin Mirallas, Tom Davies and debutant Ademola Lookman demolished City and Koeman went home a happy man.
“I am really pleased with the team performance,” he said.
“It is a big day, a big win and a well deserved one. In the second half we were perfect in every aspect.
“We played well offensively and defensively, and had good organisation in the team. We were aggressive, and when we won the ball we played forwards and scored four goals.
“At half time, we spoke about high pressing because we knew City liked to build up from the back.
“You need to show your pressing and we did that really well. There were a few moments where they played through the lines and in the first half they had some good chances.
“But in the second period, that was the type of team performance you need to beat the big sides.”