Mendenhall tore the anterior cruciate ligament in his right knee during the Steelers' regular-season finale, NFL Network insider Jason La Canfora confirmed Monday, and is out for the season.
Steelers coach Mike Tomlin said after Sunday's 13-9 victory over the Cleveland Browns that he believed the running back "potentially" tore his ACL. The Pittsburgh Post-Gazette first reported that Mendenhall had torn the ligament but suffered no other damage in the knee.
Mendenhall, who finished the season with 928 rushing yards and nine touchdowns, will have surgery before the end of January.
The Associated Press contributed to this report.