Al Horford (health and safety), Marcus Smart (foot) won't be available for Boston Celtics' series opener against Miami Heat

MIAMI — Boston Celtics starters Marcus Smart (midfoot sprain) and Al Horford (health and safety protocols) will miss Game 1 of the Eastern Conference finals against the Heat in Miami, the team announced about two-and-a-half hours before tipoff Tuesday night.

Smart suffered the sprain in Game 7 against the Bucks on Sunday, and had been listed as questionable for the series opener in Miami.

Horford's absence, meanwhile, comes as a total surprise, with no indication of any issue with the veteran forward until the team announced that he had entered the protocols. Celtics coach Ime Udoka said he initially found out about Horford's status around 5 p.m. Tuesday. He wouldn't say if Horford tested positive.

«As always, we don't comment on the status of our guys,» Udoka said. «He's feeling fine. We'll go from there. Wait to see results and tests and future tests.»

Sources told ESPN's Adrian Wojnarowski that the Celtics expect to be without Horford through at least Game 2 on Thursday night.

«It's unsure,» Udoka said, when asked how long Horford might be out. «It's more of a medical question, honestly. But there are different protocols and tests that have to be passed, and we'll know going forward, but it's not a definite that he's out for two games.»

This marks the third time Horford has been placed in the league's health and safety protocols. He tested positive for COVID-19 during the preseason, and then was placed in the protocols again in December.

It means the Celtics will require more from Robert Williams III, who missed Games 5 and 6 against Milwaukee because of a bone bruise in his left knee — the same one he had surgery on late in the regular season, forcing him to miss three-plus weeks. He was also active for Game 7 of the