Boston Celtics guard Marcus Smart. David Butler II-USA TODAY Sports

Celtics starting point guard Marcus Smart and starting big man Al Horford will miss the first game of their Eastern Conference Finals matchup against the Heat Tuesday night, the Celtics announced.

Smart, the 2022 Defensive Player of the Year, had previously been considered questionable with a right mid-foot sprain. Reserve guard Derrick White could get the starting nod in Smart’s absence.

The absence of Horford is a surprise. The Celtics have revealed that the 35-year-old veteran, a former five-time All-Star, has entered the NBA’s COVID-19 health and safety protocols. Horford was a crucial part of the Celtics’ interior attack against the Bucks during the Conference Semifinals. Sources tell Adrian Wojnarowski of ESPN that the Celtics are anticipating that they will be without Horford at least through Game 2 of the series Thursday.

Starting center Robert Williams, who has not played since Game 3 of the Conference Semifinals for Boston due to a knee injury, will be available to play. Reserve forward Grant Williams turned in a stellar Game 7 in the place of Robert Williams, and could get the starting nod as Horford’s replacement tonight. Wojnarowski tweets that Williams underwent some encouraging team practices and will be able to log major minutes against the Heat.

Miami is not without its own injury issues heading into the series. The Heat will also be missing their starting point guard, former six-time All-Star Kyle Lowry, as he continues to deal with a hamstring injury. Guards Max Strus, Gabe Vincent and Caleb Martin have all been listed as questionable ahead of tonight’s contest, as has starting power forward P.J. Tucker.

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