The Detriot Lions have placed quarterback Matthew Stafford on the reserve/COVID-19 list.
Stafford did not take part in an open practice session on Wednesday ahead of the Lions' encounter with the Minnesota Vikings on Sunday.
Detroit later revealed he had been added to the reserve/COVID-19 list, but did not state whether he has tested positive for coronavirus.
That announcement came a day after linebacker Jarrad Davis was placed on the list.
The #Lions have placed LB Jarrad Davis on the Reserve/COVID-19 list. pic.twitter.com/sDh4ZNfygh
— Detroit Lions (@Lions) November 3, 2020
A Lions statement said: "The 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, their club is required to immediately place the player on the reserve/COVID-19 list. Per agreed upon NFL-NFLPA policy, clubs are not permitted to comment on players' medical status other than referring to roster status.
"Clubs may not disclose whether [a] player is in quarantine or is a positive for COVID-19."
Stafford was also put on the list in August, but it emerged he had a false positive test.