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.
Smart, who missed Game 1 because of a midfoot sprain that he suffered in Game 7 against the Milwaukee Bucks, was listed as probable to play. Smart played 40 minutes and had 24 points, 12 assists and nine rebounds while Horford had 10 points in 33 minutes.
"[Horford] passed all the protocols, passed all the tests," Celtics coach Ime Udoka said before the game. "I'm not a doctor, but he had to pass a certain amount of tests and you know, league protocols, and so he did and he was feeling fine from the get-go. Happy to have him back."
For a player to return to game action after testing positive for COVID-19, 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 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 game-day 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 he returns two consecutive "not detected" results from PCR tests collected on game day.
If either PCR test returns a positive or inconclusive test result, this protocol shall not apply, and the player must remain in isolation.
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."