Littrell was suspended 50 games after testing positive for a drug of abuse for the second time, Jenifer Langosch of MLB.com reports.
The 24-year-old started last season in Double-A but pitched well enough to earn him a call-up to Triple-A Memphis midway through the season. His ERA jumped from 1.72 to 4.56 with the switch, so it was likely he was going to need more time in the minors regardless.