Detroit Lions quarterback Matthew Stafford has been placed on the Reserve/COVID-19 list, per Ian Rapoport of NFL Network.
Placement on the list means a player either (a) has tested positive for COVID-19 or (b) has been quarantined after "having been in contact with an infected person or persons," per Jordan Dajani of CBS Sports:
"The new reserve list category was created for a player who either tests positive for COVID-19 or who has been quarantined after having been in close contact with an infected person or persons. If a player falls into either of these categories, his club is required to immediately place the player on the reserve/COVID-19 list.