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. Clubs may not disclose whether a player is in quarantine or is positive for COVID-19."

The reason for Stafford's placement on the list has not been disclosed.

Dave Birkett of the Detroit Free Press outlined the Lions' recent COVID-19 testing schedule:

"Lions took COVID tests Tuesday and Wednesday, another one Friday. Presumably, Stafford's Friday test came back positive. Today was the day players were allowed into the building for physicals, equipment...

"Or Stafford was around someone who tested positive in the last few days."

Stafford, whom the Lions took No. 1 overall in the 2009 NFL draft, has played 12 seasons since playing college ball for Georgia.

Per Birkett, the Lions declined comment on the matter.

Stafford, whom the Lions took No. 1 overall in the 2009 NFL draft, has played 11 seasons since playing college ball for Georgia.

He tossed 19 touchdowns and completed 64.3 percent of his passes in an injury-shortened 2019 season that lasted eight games because of non-displaced fractures in the upper thoracic spine, per Grant Gordon of NFL.com.

A running list tracking players who (a) landed on the COVID-19 list or (b) have opted out because of COVID-19 concerns reveals that all but three teams have players who fall into either category. At least eight Lions are on the list after Stafford's placement, per Birkett.

A diagnosis for him or someone in his family brings a heightened sense of concern. The signal-caller is married with four children, one of whom was born in June.

Stafford's wife, Kelly, was diagnosed with had an acoustic neuroma (benign) brain tumor in 2019. She then underwent a lengthy and arduous recovery process that included relearning how to walk, per Marlowe Alter of the Detroit Free Press.

Other players who have been placed on the list include Lions wide receiver Kenny Golladay, Minnesota Vikings linebacker Anthony Barr, Vikings rookie wideout Justin Jefferson, Green Bay Packers kicker Mason Crosby and Philadelphia Eagles offensive tackle Lane Johnson.

Those who have opted out include Kansas City Chiefs running back Damien Williams, New England Patriots linebacker Dont'a Hightower and Buffalo Bills defensive tackle Star Lotulelei.