Boston Celtics coach Brad Stevens said Saturday that he is unsure the team's biggest addition this offseason, center Tristan Thompson, will be ready to play in time for Boston's season opener at home against the Milwaukee Bucks on Dec. 23.

"I think that's hard to say right now," Stevens said after the team practiced Saturday. "He's not been in any part of practice yet. He's done our pre-practice ... 2-on-2, 3-on-3, very light, simulated reads work on both ends. So that's it."

Thompson, who signed a two-year, $19 million contract with Boston this offseason after spending the first eight years of his career with the Cleveland Cavaliers, hasn't played in a competitive game since March, as Cleveland was one of the eight teams that failed to qualify for the NBA's bubble at the Walt Disney World Resort in Orlando, Florida.

That layoff only further complicates Thompson's ramping up of his activity, Stevens said, after he hurt his hamstring in the lead-up to the start of training camp.

"He hasn't played in a long time," Stevens said. "He didn't play in the bubble, so there will be a ramp-up period for him whenever he is able to go live in practice, and I'm assuming it won't be just one of those situations where you're available one day and you get a good practice in and you can play. I think it'll be a little more than that.

"I don't know what that means with regard to Dec. 23. Every day that passes I'd say that becomes more unlikely."

One player who won't be available for the season opener is star point guard Kemba Walker, as Boston has already said there won't be an update on the status of his work to strengthen his left knee until sometime in early January. Stevens said Saturday that Walker is doing some individual spot shooting with "a little bit of movement" but that he isn't doing any work in team settings, or even in small groups.

The other free agent Boston signed this offseason, veteran point guard Jeff Teague, was asked Saturday whether he's been given any indication if he will start in place of Walker to begin the season. He said it has yet to come up.

"I'm not sure," Teague said. "But it really don't matter. We've got a lot of great guys, a lot of talented players, and it's going to be a whole team effort not having a guy like that right now, a dynamic point guard like that, so I guess we'll figure it out."