In a surprise announcement, the Boston Celtics entered Al Horford into the NBA’s concussion protocol program and ruled the four-time All-Star out of the team’s nationally televised Eastern Conference clashes with the unbeaten Chicago Bulls and Cleveland Cavaliers on Wednesday and Thursday night.

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According to the team’s press release, Horford was hit in the head during Monday’s practice, but was cleared to practice on Tuesday after showing no ill effects. However, symptoms arose during practice on Tuesday, when Horford was removed from the court and evaluated by team physicians that night.

When asked about Horford’s absence at the end of practice on Tuesday, Celtics coach Brad Stevens told the media Horford “headed home sick,” according to The Boston Globe’s Adam Himmelsbach.

Horford has since been placed in concussion protocol and must be cleared medically to return. The team ruled him out for Wednesday’s home game against the Bulls, and Stevens soon confirmed during his appearance on 98.5 The Sports Hub that Horford would miss Thursday’s game in Cleveland.

Per league guidelines, “If a player is diagnosed with a concussion, he will not return to participation on that same day or the next day.” Horford cannot begin the NBA’s “return-to-participation exertion protocol” until 24 hours after Tuesday’s diagnosis. In order to be cleared, Horford must be evaluated by a team physician, complete the return-to-participation protocol — riding a stationary bike, jogging, agility work and non-contact team drills, all without any symptoms at each step of the process — and get the team’s medical clearance approved by NBA concussion program director Dr. Jeffrey Kutcher.

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Horford has averaged 12 points, 5.3 rebounds, 4.7 assists and 2.3 blocks in his first three appearances for the Celtics. Boston has not played since Saturday and will have another two days off following the back-to-back against the Bulls and Cavaliers. Obviously, Horford’s status for Sunday’s game against the visiting Denver Nuggets depends on the severity of the head injury he suffered at practice.

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Ben Rohrbach is a contributor for Ball Don’t Lie and Shutdown Corner on Yahoo Sports. Have a tip? Email him at rohrbach_ben@yahoo.com or follow him on Twitter! Follow @brohrbach