Marcus Smart (midfoot sprain) and Al Horford (health and safety) missed Game 1 in Miami.
The team made the announcement two-and-a-half hours before tipoff.
Smart sprained his ankle in Game 7 against the Bucks and was questionable for Game 1 in Miami.
Horford's absence is a total surprise, as the team gave no indication of a problem until he entered protocols.
Horford's status was first revealed to Celtics coach Ime Udoka around 5 p.m. Tuesday. If Horford tested positive, he wouldn't say.
Udoka: "We don't comment on our guys' status." "He's okay. Then what? Wait for results and tests."
Adrian Wojnarowski of ESPN reports that the Celtics expect Horford to miss Game 2 on Thursday.