MIAMI -- The Boston Celtics announced Thursday afternoon that Al Horford would be available to play in Game 2 of the Eastern Conference finals against the Miami Heat.

Horford, who abruptly went into the NBA's health and safety protocols a couple of hours before Boston's 118-107 Game 1 loss on Tuesday, was upgraded to doubtful Wednesday evening and then was rapidly upgraded from questionable to available in the span of about 90 minutes Thursday.

For a player to return to game action after testing positive, there had been two paths to a return: either recording two consecutive negative PCR tests at least 24 hours apart, or return two consecutive PCR tests sampled at least 24 hours apart with cycle threshold (CT) values greater than 30 beginning no sooner than Day 4 and 5 after initially testing positive.

However, in a memo dated April 30 and obtained by ESPN, the league added a third gameday testing mechanism to potentially allow for players to return the day of a game.

In that circumstance, a player could return to team activities if they return two consecutive not detected results from lab-based PCR tests collected on game day.

If either lab-based PCR test return a positive or inconclusive test result, this protocol shall not apply, and the player must remain in isolation.

Marcus Smart, who missed Game 1 because of a midfoot sprain that he suffered in Game 7 against the Milwaukee Bucks, remains probable to play Thursday night.

"He's better but still has to test it out pregame," Celtics coach Ime Udoka said after the team's shootaround Thursday morning. "But he's probable for tonight. When he goes before the game, we'll know from there, but he looked better in shootaround today and overall we expect him to play."

The Celtics also announced that Derrick White would miss Game 2 for personal reasons, as he and his wife are expecting a child.

"Derrick is having a baby," Udoka said. "It came sooner than expected. But we support our guys always in that situation. He had to rush home today."

White started Game 1 in place of Smart and finished with three points and four assists in 29 minutes. He is expected to be back with the team for Game 3 on Saturday night when the series shifts back to Boston.