There are two ways for NFL players to end up on the league's COVID-19 reserve list — they either test positive for the virus or report they were knowingly exposed to someone else who did.
In the case of Detroit Lions quarterback Matthew Stafford, who landed on the list over the weekend, he tested positive for the virus, according to Sports Illustrated's Albert Breer.
"The test was administered on Friday, and Stafford was asymptomatic beforehand," Breer wrote on Monday.