Sunday's contest was set to go ahead, despite the Celtics missing seven players – including star pair Jayson Tatum and Jaylen Brown – because of COVID-19 protocols, and nine in total.
But the NBA announced the postponement just hours before tip-off after a Heat player reportedly returned an inconclusive coronavirus test, leaving Miami without the required eight players to play due to contact tracing.
Heat guard Avery Bradley (health and safety protocols) had been ruled out of the game earlier in the day.
The Celtics were set to have the minimum eight players available against the Heat after Javonte Green, Semi Ojeleye, Grant Williams, Robert Williams II and Tristan Thompson were listed as outs pre-game.
Injuries had already forced Celtics pair Kemba Walker (knee) and Romeo Langford (wrist) to miss the scheduled meeting.
It comes as COVID-19 outbreaks impact the NBA following a coronavirus-hit 2019-20 campaign in the United States.
Eastern Conference leaders the Philadelphia 76ers were depleted for Saturday's loss to the Denver Nuggets after Seth Curry tested positive for coronavirus.
Contract tracing seriously impacted Doc Rivers' 76ers, who were without Curry, Tobias Harris, Matisse Thybulle and Vincent Poirier, paired with injures to All-Star duo Joel Embiid and Ben Simmons.
Philadelphia were only able to dress eight players against the Nuggets, though Mike Scott was unable to play due to a knee injury.
The Celtics own a 7-3 record in the Eastern Conference, ahead of last season's NBA Finals runners-up the Heat (4-4).
The Lakers cancelled practice on Tuesday after guard Talen Horton-Tucker was put into the league's coronavirus protocols.
It was revealed two days later that Westbrook and Bradley have joined Horton-Tucker, Dwight Howard and Malik Monk in entering the protocols.
With Westbrook and Bradley unable to travel for Friday's game against the Minnesota Timberwolves at Target Center, it has been reported the Lakers are set to sign Isaiah Thomas.
Two-time All-Star Thomas is to sign a 10-day contract to join a depleted Los Angeles roster under the hardship exemption, according to NBA insider Marc Stein and The Athletic's Shams Charania.
The 32-year-old point guard joined the New Orleans Pelicans on a 10-day contract last season, averaging 7.7 points in his three games.
Bradley has a six-year-old son who has a history of struggling to recover from respiratory illnesses and would have been unlikely to be medically cleared to enter the bubble at Walt Disney World after the first round of the playoffs, when family members will be permitted to join players.
"As committed to my Lakers team-mates and the organisation as I am, I ultimately play basketball for my family," Bradley told ESPN.
"And so, at a time like this, I can't imagine making any decision that might put my family's health and wellbeing at even the slightest risk.
"As promised also, I will use this time away to focus on the formation of projects to help strengthen my communities."
Bradley stands to lose a projected $650,000 in salary by sitting out the season's resumption.
The Lakers enter the 22-team restart with the best record in the Western Conference. Bradley has been a key two-way player this season, averaging 8.6 points and 2.3 rebounds.
The 10-year veteran joins Washington's Davis Bertans and Portland's Trevor Ariza as players who have already said they will not participate in the restart in Orlando. Bertans and Ariza, however, are on teams currently on the outside of the playoff picture.
The Lakers and the Clippers are the top two seeds in the West as things stand, boasting an array of talents after Anthony Davis joined James last year, while Kawhi Leonard and Paul George went to their rivals.
Although James' Lakers lead the conference, they had been beaten by the Clippers in the sides' prior two meetings.
That run came to an end on Sunday, however, when the four-time MVP came up with 28 points, nine assists and seven rebounds in a 112-103 road win. Davis finished with 30 points and eight rebounds, as George (31 points) and Leonard (27 points) could not keep the Clippers in the game.
Coming less than 48 hours after the Lakers had beaten the Bucks, James recognised the significance of his team sending a message to their fellow title contenders.
"It was hard fought, two teams that are getting up for the playoff," he told ESPN. "We're still trying to get better. Both teams are trying to get better.
"We won it with our defense, and we won it with our offense at times, as well.
