Celtics' Al Horford, Marcus Smart out for Game 1 vs. Heat; Horford in Protocols

The Boston Celtics will be notably short-handed for Game 1 of the Eastern Conference Finals against the Miami Heat on Tuesday.

Boston announced guard Marcus Smart will not play because of a right mid-foot sprain. His status was one of the biggest storylines coming into the series, and he will miss at least this first contest on the road.

He will not be the only Celtic absent, though, as big man Al Horford will also be out because he is in the health and safety protocols.

Smart missed Game 2 of Boston's Eastern Conference semifinals victory over the Milwaukee Bucks but returned to play a key role as the series progressed.

He scored 21 points and dished out seven assists in a critical Game 6 win on the road that kept the team's season alive and then responded with a double-double of 11 points, 10 assists and seven rebounds in the decisive Game 7.

The league's Defensive Player of the Year averaged 12.1 points, 5.9 assists, 3.8 rebounds and 1.7 steals per game during the regular season. He also does much of the ball-handling for the Celtics when he is on the floor, which allows Jayson Tatum and Jaylen Brown to find openings away from the ball.

Smart was surely expected to help slow down Jimmy Butler, Tyler Herro and others on the perimeter with his defensive prowess as well.

Horford averaged 10.2 points and 7.7 rebounds per game during the regular season but has taken on more importance in the playoffs with Robert Williams III in and out of the lineup because of a knee injury.

Fortunately for Boston, ESPN's Adrian Wojnarowski reported Williams "had a couple good days of practice and should be able to play significant minutes in Game 1."

Whether he can replicate Horford's impact from the Bucks series could go a long way toward determining Boston's chances. The veteran had double-digit rebounds in five of the seven contests and exploded for 30 points in the Game 4 win.

Any sidelined player in this Eastern Conference Finals will face something of a tight timeline seeing how each game will be played on an every-other-day basis.

Wojnarowski reported the Celtics are "prepared for the likely scenario" that Horford will remain in protocols for Thursday's Game 2 as well.