Jay Glazer / FOXSports.com The Minnesota Vikings' offense may suffer a serious blow. FOX Sports has learned that the Vikings have been informed by the league office that second-year phenom running back Onterrio Smith is facing a four-game suspension for violating the NFL's substance-abuse policy. Smith, who was slated to contribute heavily to the Vikings offensive attack this season, will not be officially suspended until after he appeals the four-game ban. Smith had first-round talent on many teams' draft boards a year ago, but he slipped due to off-field concerns. It is believed he incurred two strikes against him on the league's drug policy this year, which would support the ban. Last year as a rookie, Smith produced a solid 5.4-yard-per-rush average on 107 carries. He finished with 579 yards and five touchdowns in a rotation used in the absence of the injured Michael Bennett.