Strong safety Troy Polamalu, the catalyst of the Pittsburgh Steelers defense and a player who has emerged as one of the league's most versatile defenders, suffered an ankle injury during Thursday's practice and was forced to leave the session early. The third-year veteran, who has been selected to the AFC Pro Bowl squad in each of the last two seasons, will be listed as "probable" on the Steelers' injury report. By definition, a "probable" listing on the report means that a player is 75 percent likely to participate in that week's game. How the injury occurred is not yet known. Super Bowl practices are closed to the media, with the exception of one "pool" reporter. Polamalu told the pool reporter assigned to the Pittsburgh practice that he "tweaked" the ankle. The incident was not noted on an original practice report filed early in the evening, but it is believed the NFL was revising it. Polamalu did appear on the official injury report as having missed at least a portion of the so-called "team," or 11-on-11, drills. Sources said that Polamalu began treatments for the ankle injury immediately after the practice. It is not known if he will participate in the Friday practice.