Horford was placed in health and safety protocols and is a late scratch for Tuesday's Game 1 of the Eastern Conference Finals against the Heat, Shams Charania of The Athletic reports.
This is a huge loss for the Celtics, and Marcus Smart (foot) will be sidelined as well. It's not clear how long Horford will be out, but his status for Thursday's Game 2 looks to be murky, at best. Adrian Wojnarowski of ESPN reports that the Celtics are "prepared for the likely scenario" that Horford remains out for Game 2. His absence should push Robert Williams and Daniel Theis into more prominent roles. Williams didn't play in Game 7 of Round 2 against the Bucks despite being listed as available. However, coach Ime Udoka said Williams has no restrictions moving forward.