The linebacker made 17 tackles during the team's 19-16 win over AFC North rivals the Baltimore Ravens at the weekend, but played the fourth quarter with a fracture in his left hand.
Head coach Mike Tomlin told reporters: "Lawrence Timmons broke his left hand. That's been casted. He is ready to go. That happened late in the third or early in the fourth quarter of the game.
"He was able to finish. We fixed it. He has full mobility in all of his fingers, but it has been secured with a cast."
Timmons has 51 tackles and an interception so far this season.