Before the 2006 season commenced, Molina participated in the inaugural World Baseball Classic (WBC) for Puerto Rico. After returning to the Cardinals, he changed his jersey number from 41 to 4. However, the regular season presented some of his greatest offensive challenges as he struggled through a career-worst .216 batting average in 461 regular-season plate appearances. It was a culmination of a decline over his first three seasons; Molina's on-base plus slugging percentages (OPS) registered at .684 in 2004, .654 in 2005 and .595 in 2006. The low batting average was due in part to a deflated batting average on balls in play (BABIP) of .226 (normal is around .300), a career low.