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About 90 minutes before the Celtics faced the Knicks on Friday at TD Garden, forward Al Horford was breathing heavily as he completed drills with strength and conditioning coach Bryan Doo. He sprinted the length of the court and back several times. He shuttled sideways while hitting basketballs back to Doo.

Horford has been sidelined since suffering a concussion on Oct. 31, but his return appears to be near.

“I’m getting better each day, and the past two days have been really good for me,” Horford said. “I was able to get on the court a lot more and run up and down, do a lot of movement, a lot of stuff I wasn’t able to do a couple of days ago.”

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Horford did not play against New York, but he said he will join the Celtics on their upcoming road trip to Indiana and New Orleans. Earlier, coach Brad Stevens said Horford would only join the team if there was a chance he could play, so his presence will be an encouraging sign.

Horford declined to describe how he was initially injured at practice, or to discuss what symptoms he has experienced, but he said his recent progress has given him hope. The forward said he had been hit in the head before, but that he had never experienced a concussion like this one.

“With an ankle, with a knee, I’ve played through plenty of stuff and it’s no problem,” Horford said. “But with the brain, it’s a little different. It’s been a challenge for me because I’ve had to be patient. The good thing is that now I’m at the point that I’m starting to feel good again.”

In addition to advancing through the league-mandated concussion protocol, Horford will also have to work to get into game shape, because he was only recently able to resume cardiovascular exercises.

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The Celtics have struggled without the four-time All-Star. Entering Friday’s game, the team had gone just 1-3 in his absence.

“It’s been difficult just because you want to be out there helping your teammates, fighting every night,” Horford said. “It’s tested my patience this time. Everyone here has been very supportive, starting with Coach. Just down the line all the guys. But I’m just very encouraged by my work today. I feel like I’m in pretty good shape.”

Stevens shakes up lineup

Stevens shook up the starting lineup for Friday’s game against the Knicks. With Horford and Jae Crowder (ankle) still out, Kelly Olynyk replaced Tyler Zeller and Marcus Smart got the call in place of rookie Jaylen Brown.

“With Marcus and Kelly, I think that, from my evaluation of [Wednesday] night, I thought they were probably our two best defenders in the game,” Stevens said. “I’d like to start off with that mindset right out of the gates.”

Olynyk made his season debut on Wednesday after missing the first six games of the year while recovering from offseason shoulder surgery. But Stevens said that since Olynyk is the second most-tenured Celtic and is familiar with the system, he is comfortable putting him into a larger role so quickly.

Porzingis heats up

After a quiet start to the year, Knicks second-year big man Kristaps Porzingis entered Friday averaging 25.3 points over his last three games.

New York coach Jeff Hornacek said that when he coached against Porzingis last year, he tended to drive through the lane and attempt wild, off-balance shots because he was being pushed around.

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“I don’t see that this year,” Hornacek said. “When he drives he’s been pretty solid, so I think his strength he worked on over the summer helped him. He’s making his shots and guys are looking for him. If he gets the ball anywhere in the lane, he should be able to get a shot off.”

Betts in attendance

Red Sox right fielder Mookie Betts attended Friday’s game. He greeted several players in the Celtics locker room prior to the game. Betts underwent minor surgery on his right knee on Thursday but was walking without assistance.


Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow


him on Twitter @adamhimmelsbach