Mathieu is vaccinated, according to a source.
To be moved off the reserve/COVID-19 list and cleared to play, Mathieu will need to test negative twice within a 24-hour period and be asymptomatic.
The Chiefs have only three other safeties on their 53-player roster following Tuesday's cuts. But Mathieu has time to be available before the Chiefs open the regular season on Sept. 12 against the Cleveland Browns at Arrowhead Stadium.
The Chiefs signed wide receiver Marcus Kemp, who was released on Tuesday, to take Mathieu's roster spot.
NFL Network was first to report Mathieu's positive test.