According to mmafighting.com:
Josh Koscheck has been forced to pull out of his UFC 109 bout against Paulo Thiago due to an undisclosed injury. The bout was scheduled to take place on Feb. 6.
At this time, it is unknown how long Koscheck will be out for. Thiago, meanwhile, will remain on the card, however, the UFC has yet to sign a replacement to fight the Brazilian.
UPDATE: Mike Swick will step in for his teammate and face Paulo Thiago at UFC 109