Last season's Champions League victory was built on solid defence, but Roberto Di Matteo has introduced a more expansive style this season.
The Italian has been rewarded with eight goals in three straight league victories, but his side were picked off at will by Atletico Madrid in a Super Cup thrashing in Monaco.
And Mikel said: "It is a kick up the backside. We have to keep our heads up and make sure it doesn't happen again.
"Sometimes it's not just about playing fancy football, it's about putting a shift in and everyone defending together.
"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, keep the shape."