Why the 49ers lost their starting QB

Facebook friend Tyler thought the San Francisco 49ers erred in asking rookie running back Anthony Dixon to help in pass protection on a second-and-20 play from its own 10-yard line Sunday. Carolina Panthers defensive end Charles Johnson knocked quarterback Alex Smith from the game with a shoulder injury on the play.

Mike Sando: Teams make substitutions for various reasons and sometimes out of necessity. In this case, Frank Gore had gained 13 yards up the middle on the previous play. A penalty nullified the play, but Gore could not recoup the energy expended during the run. He headed to the sideline. Dixon replaced him on second-and-20.

The 49ers lined up in the I-formation with Dixon behind fullback Moran Norris, both wide receivers to the weak side and tight end Vernon Davis in a three-point stance next to right tackle Anthony Davis. Vernon Davis released as a receiver and was not part of the blocking scheme.

Carolina brought five rushers, with the weak-side defensive end dropping into coverage. The 49ers had seven in protection, but with Norris picking up a linebacker blitz up the middle, Dixon and Anthony Davis were left alone in protection on the strong side while Smith turned away toward his receivers. At this moment, the 49ers were trusting their quarterback's health to a pair of rookies while the offense was backed up near its own end zone. One of those rookies, Dixon, had hardly played.

Johnson, the Panthers' strong-side defensive end, beat Anthony Davis to the outside. Panthers safety Charles Godfrey lurked near the line of scrimmage on the same side, drawing Dixon toward him. Anthony Davis retreated in a futile attempt to recover, but he had no chance at making the block on Johnson. Dixon couldn't help because he had drifted forward toward Godfrey near the line of scrimmage.

Smith never saw Johnson coming. Johnson sacked Smith and landed on him, driving Smith's left shoulder into the ground.

Assigning blame for blocking miscues is risky without knowing each player's assignment. In this case, Dixon conceivably could have followed his assignment to the letter. It's also possible he erred. Either way, the 49ers' best-case scenario would have included Anthony Davis making his block cleanly. Short of that, Dixon ideally would have peeled back and blocked Johnson, recognizing a charging defensive end as the greatest threat.

Neither of those scenarios materialized.