Pittsburgh Steelers quarterback Ben Roethlisberger was carted off the field after suffering an apparent knee injury against the St. Louis Rams and looks to have an MCL injury that will likely keep him out against the Baltimore Ravens Thursday, according to former NFL physician David Chao.

UPDATE, 5:55 p.m.: ESPN's Adam Schefter reports that the injury will sideline Roethlisberger for a minimum of four weeks.

The MCL injury would be considered the best-case scenario, which could keep him out two weeks minimum, according to Chao. The worst-case scenario could be an injury related to the patellar tendon or the anterior cruciate ligament (ACL), which could lead to significant time missed.

Roethlisberger was helped to the sideline before being carted off the field in the third quarter. The Steelers were leading the game 9-3 when Roethlisberger was hit low by Rams safety Mark Barron.

The Steelers have announced that Roethlisberger will not return for the rest of the game, according to Michael David Smith of Pro Football Talk. Roethlisberger had thrown for 192 yards and an interception before being injured.

Michael Vick has stepped in as the Steelers quarterback. Vick played last season with the New York Jets, but only attempted 121 passes in limited playing time. Vick will likely need to adjust to Steelers offensive coordinator Todd Haley's system quickly.

The severity of Roethlisberger's injury is not known for certain at this time, but it looks as though he will not be back for Thursday's game against the Baltimore Ravens.