Concussion was an ordeal for Al Horford

During his convalescence, even watching TV or riding in a car was painful for Al Horford.jim davis/globe staff/file 2016/Globe Staff

ORLANDO — As the Celtics navigated through the start of this season, their new cornerstone player was sitting in darkness in his Newton home, wearing sunglasses and watching his teammates play.

Forward Al Horford, perhaps the most important free agent signee in this franchise’s history, suffered a concussion when he took an inadvertent blow to the head during a practice on Halloween.

For three scary, confusing, and frustrating weeks, he was essentially paralyzed by the unknown while the Celtics plodded along without him. Slight progress was consistently followed by a more severe setback. In an interview with the Globe Tuesday, Horford recounted how a seemingly harmless practice incident evolved into something more harrowing.

“The thing about it was,” Horford said, “the symptoms just kept getting worse.”

Horford initially felt fine after taking a hit during that Monday practice. He completed a photo shoot with the Globe that afternoon and had no issues when he went to practice Tuesday. Late in that session, though, he began to feel woozy and nauseous. He was sent home and entered the NBA’s concussion protocol.

Horford was told not to read or use his cellphone or watch television, and he was to avoid all strenuous activities. He mostly slept, drank water, and watched his baby son Ean play. Doctors told Horford that outdoor walks could help, so he strolled around his neighborhood.

“It was all a very weird time for me,” Horford said, “because, like, what do you do when you can’t do anything?”

Horford’s no-television rule had one exception. He was devastated he could not help his new teammates, so he had to at least watch them play. He would turn out all the lights in his living room, put on his sunglasses, and hope for the best.

Horford had headaches and nausea, and he was bothered by light, noise, and motion. For Horford, the experience was especially concerning because he had no point of reference. He said he took a similar shot to the head in high school and had concussionlike symptoms, but was fine the next day. The same thing happened about four years ago, when he played for the Atlanta Hawks. This time, he seemed to be regressing.

“As the days kept going by, I started to feel worse,” he said. “Just a lot of things that wouldn’t go away. It was a lot of stuff I didn’t really understand, because it’s never happened to me before.”

After about two weeks, Horford began to work out with the team’s training staff. He hoped to at least support his teammates from the bench.

“Then I noticed that just being in the arena, these lights, would kill me,” he said. “They would bring my symptoms back. I felt like I was close to fainting. The noise and the lights would bring everything back to square one.”

Michael Dwyer/Associated Press/File 2016

So Horford would watch the Celtics while sitting in a quiet and dark room in the bowels of TD Garden, so close yet still so far away. The team lost three of its first four games without Horford, and that added to his urgency.

“It was hard for me,” he said. “There was a point where I was like, ‘Screw it, I’m going to try to come in early.’ So I went on the Indiana and New Orleans trip hoping that I could come back.”

Horford missed the game against the Pacers Nov. 12 before having a great workout in New Orleans. After that session, his symptoms returned. He was sent back to Boston before the Nov. 14 game.

Celtics coach Brad Stevens consistently reminded Horford that he was expecting his second child, and that his health was far more important than basketball. Horford sought advice from current and former players who have suffered concussions. A conversation with Bulls forward Doug McDermott was most essential.

McDermott was concussed on the same day as Horford, during Chicago’s win over the Nets. He missed just one game and returned Nov. 4. But on Nov. 12, McDermott suffered another concussion in a win over the Wizards. He has not played since then. Horford said McDermott’s experience gave him more pause.

“Concussions, you have to be really honest with yourself, and sometimes that’s hard to do,” he said. “As athletes, we’re always trained to play through stuff. And I’ve played through all types of injuries — ankle sprains, shoulder, whatever — and with the brain it’s just different. That was the hard part for me to understand.”

Eventually, Horford’s symptoms completely subsided and he passed through the NBA’s protocol. Since returning Nov. 19, he has averaged 16.8 points, 6.9 rebounds, 5.1 assists, and 2.8 blocks per game, and the Celtics have gone 6-3.

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Point guard Isaiah Thomas missed Tuesday’s practice because of a mild groin strain. His status for Wednesday’s game against the Magic is unclear. The Celtics called up rookie point guard Demetrius Jackson from the Maine Red Claws.