McKinley was a first-round pick by the Falcons in 2016. He had 17 1/2 sacks, two passes defensed, two forced fumbles and one fumble recovery in 49 career games. He had one sack in four games this season.
The 49ers also placed three more players on the reserve/COVID-19 list with receiver Brandon Aiyuk, left tackle Trent Williams and tight end Daniel Helm being added.
Defensive lineman Arik Armstead tested positive for the coronavirus and went on the list Monday. Defensive tackle Javon Kinlaw, offensive lineman Hroniss Grasu and linebacker Joe Walker were also added to the list earlier this week.