The Chiefs returned home to host the Steelers in an attempt to create the biggest upset of the week and get their first back-to-back wins since 2007.  Optimism grew quickly as Jamaal Charles ran the opening kick 97 yards for a touchdown, starting the game with a 7-0 Chiefs lead. The Steelers dominated the rest of the first half with 20:45 time of possession, going into halftime with a 17-7 lead.  The Chiefs' defense began to pressure Ben Roethlisberger in the third quarter, forcing two interceptions (both by Andy Studebaker in his first NFL start).  By the fourth quarter, the Chiefs tied the game up at 17-17.  The fourth quarter resulted in a touchdown each, sending the game into overtime. The Steelers won the toss in overtime, but failed to get within field goal range.  Of note, Roethlisberger suffered a knee to the head concussion and was replaced by Charlie Batch.  The Chiefs took over at the 20 and, after a 61-yard catch and run by Chris Chambers, Ryan Succop kicked the winning 22-yard field goal.  With the win, the Chiefs improved to 3-7 and snapped a 10-game home losing streak, the longest in team history.
Question: What injury caused Charlie Batch to replace Roethlisberger ?
Answer: knee to the head concussion