Miller slipped to the ground and suffered a concussion on Wednesday morning, the report says.
Miller, who has been placed on the team's reserve/non-football injury list, missed the entire 2013 season after suffering a concussion in Browns training camp and reportedly considered retiring from football. The 25-year-old had been listed as part of the Chargers' 53-man roster.
On Wednesday, Chargers coach Mike McCoy reportedly declined to say whether Miller had been hospitalized.
A fifth-round pick in the 2012 draft out of Colorado, Miller appeared in eight games for Cleveland in 2012.