MIAMI (AP) — Boston was without two starters for Game 1 of the Eastern Conference finals on Tuesday night, with Al Horford entering the NBA’s health and safety protocols and Marcus Smart unable to play because of a mid-foot sprain.

The Celtics announced those updates about three hours before the series opener against the Miami Heat, which Boston dropped in Miami, 118-107. Derrick White was added to the starting lineup in Smart’s place, and Grant Williams was starting in place of Horford.

Boston was already bracing for Smart — the league’s reigning defensive player of the year — to be out, after he injured his foot on Sunday in Game 7 of Boston’s East semifinals win over Milwaukee. But losing Horford was a surprise blow to the Celtics, adding to the team’s lineup dilemma.

Celtics coach Ime Udoka said he initially found out about Horford’s status around 5 p.m. Tuesday, or roughly 3 1/2 hours before tip-off. He would not 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.”

Horford averaged 13.0 points and a team-best 10.4 rebounds in the Celtics’ seven-game win over Milwaukee, starting all those contests. This is the start of Horford’s third stint in the protocols this season; he missed time in the preseason and Boston’s season opener, then missed five more games in December.

#NEBHInjuryReport update:

Al Horford (Health & Safety Protocols) – OUT
Marcus Smart (right mid-foot sprain) – OUT

— Boston Celtics (@celtics) May 17, 2022

Typically, it has taken players who test positive a minimum of five days to escape the protocols. That would bring Horford’s availability for Game 2 in Miami on Thursday into question — and possibly longer. The teams will play every other day in this series; the Celtics will play host to Games 3 and 4 on Saturday and Monday.

But Udoka said it wasn’t certain Tuesday that Horford would miss Game 2, though noted there are “different protocols and tests that have to be passed.”

“I feel like this is the world we’ve been living in for a long time, and certainly this year,” Heat coach Erik Spoelstra said. “Every team has had a lot of experience dealing with this. This has become kind of the normal prep: Expect the unexpected. We just have to focus on bringing our best version tonight. It’s the playoffs. You’re going to get great competition.”

Smart played in six of the seven games against Milwaukee. He missed Game 2 because of a quad injury. Then came the foot issue, causing the Celtics to originally list Smart as questionable on their injury report — though Udoka cautioned after the team’s morning shootaround Tuesday that he was was still dealing with considerable soreness.

“The soreness was too much,” Udoka said, adding that Smart is also dealing with swelling. Smart will continue receiving treatment and be re-evaluated Wednesday, Udoka said.

The Celtics entered Tuesday 5-8 without Horford this season and 5-7 without Smart.

“We’ve unfortunately had quite a bit of practice with this early in the season due to injury and COVID situations,” Udoka said. “Other guys have to step up and we’ll do the same with these guys out.”

Miami is still missing one starter: The Heat had already ruled point guard Kyle Lowry out for Game 1 because of a hamstring injury. Lowry has now missed seven of Miami’s last nine games because of that problem.