The holders blew a two-goal lead at Stamford Bridge, with a pair of defensive blunders costing them dearly against the Italian champions.
Mikel needlessly gave the ball away 10 minutes from time and then failed to prevent Claudio Marchisio playing Fabio Quagliarella into space for the equaliser, with David Luiz also all at sea.
"If I want to make up an excuse, I can," Mikel said. "But, for me, I gave the ball away. I put my hands up, we move on.
"I'm sorry about the mistake, but there are no excuses."
Manager Roberto Di Matteo admitted Chelsea only had themselves to blame and Mikel added: "I think that's what you get from playing in the Champions League.
"You make a mistake, you get punished for it. I think we looked so good all throughout the game, except when it was 2-2. We just lost our head."