Based on how effective the 49ers' passing offense was in 2012, and how it has slumped in 2013, it could be argued that wide receiver Michael Crabtree is the most valuable player in the NFL.Last season, the 49ers averaged 206 yards passing per game. This year, the average has fallen to 168 yards per game – the worst in the league. In 2012, quarterback Colin Kaepernick averaged 8.3 yards per pass attempt; this season, the number has dropped to 7.1.The missing element? Crabtree.In his fourth season in the NFL, Crabtree had a sensational season, with career highs in catches (85), yards (1,105) and touchdowns (9). Whether it was catching passes from Alex Smith in the first half of the season or Kaepernick in the second half, Crabtree was the 49ers' go-to receiver.This season, the 49ers have missed his ability to get open, stretch the field and open up other offensive options. Though wideout Anquan Boldin and tight end Vernon Davis have been terrific, the 49ers have missed the Kaepernick-to-Crabtree chemistry that at times was electric last season. In the final five regular-season games of 2012, Crabtree three times had more than 100 yards receiving and averaged seven receptions....