The Blues midfielder turned his back on the PSG forward's effort from outside the box, with the ball flicking off the Nigerian and past a helpless Thibaut Courtois in the Chelsea goal.
Mikel would atone for his error by scoring what could prove a crucial away goal at the other end although Ballack was not impressed with the 28-year-old for his role in the opener.
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"You have to stay like a man in the wall and don't turn away because you don't know what's going on. If he passes, the game has to carry on. You can't turn away."
Rio Ferdinand added: "He'll look back on this and he'll be embarrassed at the way he's turned his back. Most players have done this at some point in their career.
"When you get punished like that and it gets highlighted, there's no place to hide. He's just fortunate he went up the other end and scored. You've got to stay strong and really want to take the ball in the face."