Fitzpatrick is the second Steelers player in three days to land in COVID-19 protocols, joining quarterback Ben Roethlisberger, who tested positive Saturday night and missed Sunday's 16-16 tie against the Detroit Lions.
The positive tests puts Fitzpatrick's status for Sunday night's game against the Los Angeles Chargers in doubt.
Roethlisberger, who is vaccinated, could return for Sunday's game. According to the NFL's COVID-19 policy, a vaccinated player with a positive test can return if he has two negative PCR (polymerase chain reaction) tests taken at least 24 hours apart and he is asymptomatic for 48 hours.
Unvaccinated players must quarantine 10 days after a positive test and can return if they are asymptomatic, according to the NFL's protocol.
Coach Mike Tomlin said Roethlisberger was symptomatic Saturday, prompting him to get tested.
Missing Fitzpatrick for Sunday's game against the Chargers would be another blow to a defense that's already dealing with injuries to cornerback Joe Haden and All-Pro pass rusher T.J. Watt. Haden sustained a mid-foot sprain against the Lions and didn't return, while Watt left with a hip and knee injury after a third-quarter sack of Jared Goff. MRIs of Watt's hip and knee came back negative, a source told ESPN's Adam Schefter. He is sore, but it's uncertain how much time -- if any -- he'll miss, per Schefter.
The Steelers officially put Fitzpatrick on the reserve/COVID-19 list later Monday. The team also signed quarterback James Morgan to their practice squad. To make room, nose tackle Isaiah Mack was released.