Brentford raced to a 3-0 lead in the first-half thanks to Bryan Mbeumo’s goal and a brace from Ollie Watkins.
The Rams improved after the break but the damage was already done by then and Cocu’s men suffered their second league defeat of the season.
And after the game, Cocu didn’t hold back – labelling his players ‘arrogant’.
Cocu said: “I’m mostly disappointed with the attitude we had in all of the first-half.
“It’s like an arrogance based on nothing – we have to work hard because we’re not in a position where we’re top of the league so your attitude and press you show on the pitch is really important.
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“But if you don’t start well then it’s really hard to play good football and there were so many mistakes on the ball.”