Smith suffered a concussion in the first half of Sunday's 24-24 tie with the Rams according to ESPN.com.
He completed 7-of-8 passes for a touchdown before leaving the game and handing over the keys to the offense to second-year backup Colin Kaepernick.
Say what you want about Smith's shaky first five seasons in the NFL, but since the arrival of Jim Harbaugh, Smith has become the poster boy for how to have success while flying under the radar.
The former No. 1 overall draft pick in 2005, Smith is completing a career-high 70 percent of his passes in 2012 and is on pace for career-highs in passing yards and touchdowns this season.
Can San Francisco reach the Super Bowl with Colin Kaepernick at QB?
Although the Niners are a team built to win by running the ball effectively and playing lights-out defense, Smith has developed into the perfect complement to that style under center. He knows when to take a chance and when to throw it away, evidenced by his mere 10 interceptions dating back to the start of 2011 (25 games).
He had 53 in his first 54 games with San Francisco.
Inserting just anyone under center will not do for the 49ers. Kaepernick has shown promise in his brief professional career, but he lacks the experience and decision-making skills that Smith possesses.
Of course, San Francisco will need a healthy Frank Gore and an injury-free top-five defense heading into the postseason in order to reach the Super Bowl, but the most overlooked need for the 49ers is a healthy Alex Smith.
He has successfully made the transition from draft bust to field general, becoming an extension of Jim Harbaugh in the huddle. The 49ers don't need the second coming of Peyton Manning, just the smarter and more patient Alex Smith.
If the Niners' defense and Frank Gore are the cake, then Smith is the icing that brings it all together.
The fact that the 49ers couldn't overcome the 3-5 Rams at home on Sunday without Smith on the field proved only one thing: He is crucial to San Francisco's success moving forward.
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