Pittsburgh Steelers running back Le'Veon Bell is facing a four-game suspension for a violation of the NFL's drug policy, sources told ESPN's Dan Graziano.

The suspension is the result of missed drug tests, not a failed one, sources said. The appeals process is ongoing, which is why the suspension hasn't been announced.

No date for Bell's appeal has been set, though it's expected that it will be heard before the regular season. If it isn't overturned, Bell would miss the first four games of the season.

According to the NFL's substance-abuse policy, a player must miss multiple drug tests before he would be subject to a suspension.

Bell was suspended for the first three games of 2015, which was later reduced to two games, for a violation of the NFL's substance abuse policy after he was arrested on charges of marijuana possession and DUI in the summer of 2014.

Since Bell entered the league, his 119 yards per game from scrimmage leads all players. He caught 83 passes for 854 yards two years ago to go with his 1,361 rushing yards.

Despite Bell's potential absence, the Steelers remain second favorites to win Super Bowl, along with the Green Bay Packers, behind only the New England Patriots (13/2) at Westgate SuperBook. The Bell news did not cause any changes to Steelers' Super Bowl odds, per Jeff Sherman of the SuperBook.

Information from ESPN's David Purdum was used in this report.