Buckner's status for Sunday's key AFC South game against Tennessee is now in question after being placed on the list. The Colts and Titans have identical 7-3 records.
A player who tests positive for COVID-19 can't return to the team until at least 10 days after the start of symptoms, plus at least 24 hours after symptoms have passed, according to league rules.
Buckner, the anchor of the interior part of the Colts defensive line, has 41 tackles, 2.5 sacks, one forced fumble and one fumble recovery this season.
Buckner is the second Colts defensive lineman on the reserve/COVID-19 list. Denico Autry is on the list after being placed on it last week. Colts coach Frank Reich didn't give an update on Autry on Wednesday.
The Colts need Buckner and Autry to try to stop Titans running back Derrick Henry. Henry has rushed for at least 100 yards against the Colts in each of the past two games.