Celtics without starters Marcus Smart, Al Horford for Game 1 of East finals vs. Heat

The Miami Heat already knew it would need to begin the Eastern Conference finals without starting point guard Kyle Lowry. The Boston Celtics learned hours before Game 1 that they would also be short-handed.

The Celtics announced about three hours prior to tipoff on Tuesday night that starters Marcus Smart (right mid-foot sprain) and Al Horford (health and safety protocols) are unavailable for Game 1 at FTX Arena.

Forward Grant Williams and guard Derrick White will start in their place.

“I feel like this is the world we’ve been living in for a long time and certainly this year. Every team has had a lot of experience with this,” Heat coach Erik Spoelstra said of learning that Smart and Horford are unavailable for the Celtics just a few hours before the game. “This has become kind of the normal prep and expect the unexpected.”

Smart, who was named the NBA’s Defensive Player of the Year for this season, sprained his right foot in the Celtics’ Game 7 win over the Milwaukee Bucks on Sunday. Smart was listed as questionable on the injury report before ultimately being ruled out.

“The soreness was too much,” Celtics coach Ime Udoka said. “There’s still swelling. He had limited basketball movement. He took a nasty fall” in Game 7 against Milwaukee.

Horford, who has averaged 13 points on 52.4 percent shooting from the field, 9.4 rebounds, 3.4 assists and 1.4 blocks this postseason, entered the NBA’s COVID-19 protocols on Tuesday.

The quarantine period is five days for asymptomatic and vaccinated NBA players to return if testing data shows they’re no longer at risk to be infectious. Based on Horford’s timeline, his five-day quarantine period would run through Sunday unless he produces consecutive negative PCR tests at least 24 hours apart before then.

Horford hasn’t clearly said if he’s vaccinated, and Udoka declined to say if he tested positive or when he might return. Horford was at the team’s shootaround on Tuesday, and Udoka said he’s unsure if he exposed the team to COVID.

If Horford can’t clear protocols until Monday, he would miss at least the first three games of the series.

The only other Celtics player unavailable for Game 1 is Sam Hauser (right shoulder instability).

The only Heat player out Tuesday is Lowry.