The Boston Celtics have lost Al Horford and Marcus Smart for Game 1 of the Eastern Conference Finals against the Miami Heat.

Horford will miss out after entering the NBA's health and safety protocols, while Smart has a right mid-foot sprain, the team said.

Losing center Horford and point guard Smart just hours before the series opener is a major blow to head coach Ime Udoka.

It remains to be seen how long the pair must stay sidelined. Both played significant roles in Boston edging out the Milwaukee Bucks in a semi-final series that went to a Game 7 decider on Sunday.

Speaking before the twin absences were announced, Udoka said the Celtics would look to make a strong start against Miami, despite lacking time for a breather between series.

Udoka said: "We didn’t get a ton of rest, as opposed to Miami, who had a few days off. But we're riding some momentum and staying sharp and there's benefits to that as well. Miami got [four] days off, but as far as what we did against Milwaukee, some carryover could be good."

Boston are not alone in being without key personnel. It had previously been announced Miami guard Kyle Lowry will miss Game 1 with a hamstring injury.