The Blues midfielder hit out after the embarrassing 4-1 defeat to the Spanish side in Monaco on Friday night.
Mikel, 25, insisted: "It is a kick up the backside. We have to take it. We have to keep our heads up and make sure it doesn't happen again."
Chelsea's new style has forced Mikel to re-invent his game and he admitted he is finding it hard.
He said: "Last year, we were very compact, we would fight for each other and everyone defended together but this year we want to play fancy football.
"The role for me is trying to keep the balance, trying to keep the shape."