You saw the play. Pittsburgh Steelers quarterback Ben Roethlisberger suffered a left knee injury midway through the third quarter Sunday at St. Louis. Roethlisberger has a sprained MCL and will miss four to six weeks as he recovers, a team source told ESPN senior NFL analyst Chris Mortensen.

Roethlisberger was hit low on the play, so it's worth asking: Was he hurt by an illegal hit?

The contact occurred when Rams safety Mark Barron blitzed the left side of the Steelers' offensive line. Barron stumbled as he cut inside. Based on the television replay from the end zone, it appeared that Barron reached toward Roethlisberger with his left arm and ultimately barreled into his leg with the left shoulder.

In 2009, you might recall, the NFL clarified its quarterback protection rules to include below-the-knee hits from defenders on the ground. The impetus was the torn ACL suffered by New England Patriots quarterback Tom Brady.

Here's the relevant passage from the NFL rule book: "A rushing defender is prohibited from forcibly hitting in the knee area or below a passer who has one or both feet on the ground, even if the initial contact is above the knee. It is not a foul if the defender is blocked (or fouled) into the passer and has no opportunity to avoid him." The rule includes two footnotes: "(1) A defender cannot initiate a roll or lunge and forcibly hit the passer in the knee area or below, even if he is being contacted by another player. (2) It is not a foul if the defender swipes, wraps or grabs a passer in the knee area or below in an attempt to tackle him."

Barron appeared to stumble on his own and did not appear to be blocked to the ground before the contact. Barron told reporters in St. Louis that "I tripped over somebody’s feet and on my way down I caught a piece of his leg." If anything, his actions fit the description of a "lunge" followed by a forcible hit to the passer. Referee John Hussey's crew did not call a penalty. Mike Pereira, the NFL's former vice president of officiating and now a Fox analyst, said he believes that one should have been called. The NFL routinely reviews coaches' tape of all games and frequently issues fines for plays that weren't penalized during the game.

No one is suggesting that Barron's hit was dirty. But from this vantage point, it appeared to fall into the big bucket of quarterback contact the NFL tries to avoid. You can see why.