Mikael is expected to be out until March at the very least and, with Birmingham only loaning him until the winter transfer window, with an option to extend for the rest of the season, Mikael will now remain at Chelsea for the rest of the season.
Brum manager Steve Bruce revealed: "Chelsea and ourselves put that break in at Christmas time just in case Chelsea wanted to take him.
"I would not have thought that we would extend his loan deal.
"We could not see any point in it because we hear that he can't play again until next April so his season is effectively finished."
Chelsea Mad would like to wish Mikael all the best in his rehabilitation process.