"Kemba's out," coach Brad Stevens said following the team's morning shootaround at their practice facility prior to Saturday's game. "Most likely, at minimum, the next two games, maybe, and then we'll reevaluate it after that."
Walker, who spoke before the game, said it was hard for him to miss the game against the Sixers, who were trying to sweep the Celtics for the first time since the 2000-01 season. Saturday also was the first time that Al Horford played in Boston since leaving the Celtics to sign with the Sixers as a free agent this summer.
"It's tough," Walker said. "I'm a competitor. I love to play, especially against teams like Philly, who are a great team. So it's definitely a tough decision for me to sit this one out. But at the end of the day, I just have to look at the long haul. We're looking to get to the playoffs and make a run, so I've got to look at it that way."
Walker sat out of Boston's win over the Phoenix Suns two weeks ago with soreness in the same knee. At the time he underwent an MRI to see if there were any larger issues, but it came back clean.
He said there were no plans to get his knee tested again this time, and he chalked up the issue to the normal wear-and-tear that comes with an 82-game regular season -- as well as his participation with Team USA this summer.
"I don't know," Walker said, when asked if he was dealing with tendinitis. "Just sore. A lot of basketball. I played a lot of basketball from summertime to now. Just a lot of games, I mean, it's just what happens throughout the course of an NBA season."
Stevens said Walker's knee began to bother him Thursday, when he played 31 minutes in a 119-104 victory over the Golden State Warriors. He had played at least 35 minutes in each of the prior three games -- the second time this season Walker played at least that many minutes in three consecutive games.
Stevens said Saturday morning that while the Celtics don't expect it to be a long-term issue, it is something the team could have to manage moving forward.
"You could tell he didn't have his normal burst the other day," Stevens said. "I don't think it's anything that we think is a long, long-term thing, by any means, but we certainly may have to manage it."
The Celtics have made it a point to manage Walker's minutes all season -- he's averaging 32 minutes per game, his lowest average since his rookie year, when he played 27 minutes a night.
Walker, who was named to his fourth straight All-Star Game last week, will miss his sixth game of the season Saturday night -- the same number he missed in total over his last four seasons with the Charlotte Hornets, including playing all 82 games last season.
"Just got to take care of myself," Walker said, "which I've been doing to the best of my ability. But I've got to continue to do it. That's really it."
Boston started Marcus Smart against the Sixers in place of Walker. Horford, meanwhile, returned to Philadelphia's starting lineup after sitting out Thursday's loss in Atlanta to the Hawks with his own bout of knee soreness. Horford also missed Philadelphia's first trip here in December.
"A lot happened in three years, on the court and off the court," Horford said after the team's morning shootaround. "All I have is positive memories of my time here."
In addition to Walker, the Celtics were also without Enes Kanter and Robert Williams, leaving them short-handed at center against Sixers star Joel Embiid. As a result, the Celtics had rookie Tacko Fall, who is on a two-way contract, on the active roster for Saturday's game.