Horford, who was concussed when he was inadvertently struck by a teammate at practice on Oct. 31, went on Boston's recent two-game road trip with hopes of returning to action but said he is simply not ready to play.
"I'm just not where I want to be right now, but trying to work through it," Horford said after going through a light workout with his teammates Tuesday. "This is something very new to me, so it's just trying to get used to this."
Pressed on what's holding him back, Horford declined to elaborate on his progression through the NBA's concussion protocol.
"I'm not at liberty to discuss a lot of those things yet," Horford said. "On-the-court-wise, the times that I've been on, I felt good with my workouts. It's just to play, I'm just not there yet."
Asked if he might be ready to play Friday against the Golden State Warriors, Horford said simply that he's antsy to play again.
"I've been very motivated to come back. This is just something very different," Horford said. "Like I've said, I haven't dealt with anything like this before. So we're really taking it day by day. That's just all I can do right now."
Horford later added that he is "a competitor."
"I want to be out helping my team, and us growing together," he said. "But the last thing I want to do is have a setback, come in when I'm not supposed to, and set myself back for a longer period of time.
"We're trying to be as smart as we can about this. I just want to make sure that when I'm ready to go, I'm good to go."
Horford signed a four-year, $113 million contract with the Celtics this summer. As the Celtics battle injuries at the start of the season, teammates are hopeful Horford will feel good enough to return soon.
"We want him back and hopefully he'll be back sooner rather than later," Celtics guard Isaiah Thomas said. "You can't mess with the brain. You can't mess with the head. If he's not feeling like himself, then he has to wait until he is."
The Celtics have gone 3-4 in the seven games that Horford has missed. Boston also has been without Jae Crowder (ankle) for much of the same stretch, and the team's defense has suffered without the pair.
Crowder will not play Wednesday against the Dallas Mavericks, his former team, but he engaged in shooting drills after practice.