The Boston Celtics will be without forward Al Horford for at least Friday's game against the Atlanta Hawks because of a nagging knee injury, the team announced.

Boston sits at 9-9 entering Friday's game, having lost three in a row following an impressive victory over the Toronto Raptors last week. Without Horford, the Celtics will rely upon the likes of Daniel Theis and Aron Baynes to fill in up front.

Horford is third in team scoring (11.7 points per game), third in assists (6.3 PPG), and third in rebounds (3.6). He's in the third season of a four-year, $113 million contract signed with the Celtics in the 2016 offseason.

The Celtics were hit hard by injuries last season, losing All-Star point guard Kyrie Irving for the entire postseason after undergoing knee surgery. Prized free agent signing Gordon Hayward suffered a gruesome leg fracture injury in the season opener last year. Hayward didn't return and has struggled to regain his All-Star form in 2018-19.

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Credit: Russ Isabella-USA TODAY Sports

Horford has a $30 million player option for the 2019-20 season, but it'd be a surprise if he opted out, given the huge salary he'd miss out on next year. Horford has also been vocal in his love for Boston.

A five-time All-Star, Horford has helped Boston reach the Eastern Conference Finals in each of the last two seasons. He averaged 12.9 points, 8.6 rebounds and 3.2 assists for the C's last season.

What This Means

Celtics head coach Brad Stevens has done a wonderful job keeping his team together despite injuries. Even if Horford has to miss more than a game, Stevens has the depth to replace the veteran All-Star.

But the Celtics - the consensus pick to win the Eastern Conference this season - need to start picking it up soon. They sit at a mere 9-9 and are quickly falling behind in the race for one of the top three seeds in the East.

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