The thriving Pittsburgh Steelers are starting to find their rhythm after dropping their season opener, but now, they will have to continue their campaign without franchise quarterback Ben Roethlisberger. 

ESPN reported that Roethlisberger suffered a sprained medial collateral ligament (MCL) in his left knee during their Week 3 matchup against the St. Louis Rams, which they won 12–6. 

The report added that along with the MCL injury, Roethlisberger also sustained a bone bruise. 

Roethlisberger was injured in the third quarter when Rams safety Mark Barron sacked him. 

Barron was stumbling during the play but made his way to Roethlisberger, whom he hit just below the left knee. 

Roethlisberger immediately went down and grabbed the knee in pain after the mild collision with Barron. 

The 33-year-old quarterback was assisted to the sideline but was eventually carted to the locker room and never returned to the game. 

Roethlisberger finished the game completing 20 of 24 passes for 192 yards and one interception. 

He was replaced by backup Michael Vick who will also be taking the starting quarterback duties with Roethlisberger out due to the injury. 

In the game, the veteran Vick had five completions for 38 yards and was sacked by the Rams twice. 

Initial reports said Roethlisberger is expected to be sidelined for four to six weeks. In a report by NFL.com, Steelers head coach Mike Tomlin stated that his main quarterback will "be out for a number of weeks."

Tomlin also said he does not have any more details regarding Roethlisberger's condition and would be unable to give more information at least until Friday. 

Losing Roethlisberger is tough for the Steelers since he is having a great start in the 2015 season. 

In three games played, Roethlisberger had 67 completions for 912 yards with four touchdowns and only two interceptions. 

The Steelers won their last two against the San Francisco 49ers and Rams after losing in Week 1 against defending champions the New England Patriots.