Johnson was placed on the reserve/COVID-19 list Friday afternoon, Kyle Odegard of the Cardinals' official site reports.
This explains why Johnson was held out of practice Thursday and Friday for non-injury reasons, though it doesn't necessarily mean he tested positive for COVID-19, as it's possible he merely came into contact with an infected individual. The timing suggests he isn't likely to play in Sunday's game against the 49ers, though he might've been a healthy scratch even if he were available.