According to Joe Reedy with the Cincinnati Enquirer, Bengals center Kyle Cook, who returned to the lockerroom on Thursday, had surgery to repair a high ankle sprain (more severe cases involve ligament damage). He suffered his injury against the Green Bay Packers and was placed on the Injured Reserve/Recall List, allowing him to return later in the season.
While Cook recovers, the team signed Jeff Faine and found perhaps a nugget in Trevor Robinson as a backup interior lineman. Faine, who joined the team with "ridiculously bad Pro Football Focus" grades, played well considering that before the Bengals signed him, he hasn't been with a team since last March.
Reedy writes that Cook said "he should be back by midseason."