Antonio Conte's charges had looked on course for a surprise home victory after going 1-0 up, only for Barca to level late on when Christensen played a poor back-pass across his own box, allowing Andreas Iniesta to intercept and play in Lionel Messi for the leveller.
"I had done all the hard work and kept the ball in play, but then I made a very bad decision, I should have kicked it out," said Christensen, according to TV3 SPORT.
"As a centre-back, you get punished by your mistakes, however, it's the position that I've chosen to play, so I won't be dwelling too much on it and let it get to me.
"I know I made a mistake, but there is not much else to do than to learn from it and make sure it doesn't happen again."
Chelsea will now head to Camp Nou knowing that they will need to score to avoid elimination after 90 minutes.