Aitor Karanka says his Middlesbrough team didn’t heed his Diego Costa warning.
The Blues striker fired home the only goal as Chelsea took all three points at the Riverside.
And Karana says his team didn’t play to his orders on how to stop the lethal Spain striker.
“Diego Costa is one of the top scorers in the world,” he told Sky Sports.
“Before the game I said to my players you can’t lose him for one second and they lost him.”
Despite the result Karanka added: “I am really pleased with the attitude of my players.
“We knew it was going to be hard but we are in the Premier League and one mistake gets you punished.”
Costa’s goal was his 10th of the season and took Chelsea’s winning streak to six games.
They haven’t lost or conceded a goal since Antonio Conte changed to a 3-4-3 formation.