Tom Pelissero of NFL Network reports that Alexander will undergo arthroscopic surgery for the small tear in his lateral meniscus. The surgery isn't necessarily season-ending, should the Vikings advance, but it will make the cornerback unavailable for this weekend, at least.
Alexander consulted with doctors in an effort to find a way to avoid surgery, but those efforts fell short.
Minnesota is also without cornerback Mike Hughes, who was placed on injured reserve on January 3.
The 49ers are currently seven-point favorites over the Vikings.