BOSTON (CBS) — The Celtics will be without Al Horford for at least the next two games, as the center/forward has been placed in the NBA’s concussion protocol program.

Horford was hit in the head at Monday’s practice and but felt fine that evening and ahead of Tuesday’s practice session. He was cleared to practice, but the symptoms returned during the session and he was immediately removed. He was evaluated by Celtics physicians Tuesday night and placed in the league’s concussion protocol.

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“One of his teammates swiped at the ball and hit Al in the head. Right after, from everything I was told by Al and the training staff, he felt good Monday night. He came to practice Tuesday and was cleared to practice, but once we started and he was moving around, after a few drills he felt pretty nauseous,” Celtics head coach Brad Stevens told 98.5 The Sports Hub’s Zolak & Bertrand on Wednesday. “He’ll be out the next two nights. We’re just re-evaluating him day-to-day.”

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Stevens said he is unsure of who will start in Horford’s place on Wednesday night.

In his first three games with the Celtics, Horford averaged 12 points, 5.3 rebounds and 4.7 assists.

The Celtics will have guard Marcus Smart back on Wednesday night, Stevens confirmed to Zolak & Bertrand.

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