Tristan Thompson’s injured hamstring could keep him out of Boston’s season-opener on Dec. 23, Celtics coach Brad Stevens said Saturday.

The 6-foot-9 center, whom the Celtics signed to a two-year, $19 million contract in November, “has not been in any part of practice yet,” Stevens said.

The lost practice time is especially concerning, Stevens said, because Thompson, who has career averages of 9.4 points and 8.7 rebounds over nine seasons, all with the Cleveland Cavaliers, hasn’t played an NBA game since March.

The Cavaliers, last in the Eastern Conference, didn’t qualify for the pandemic-induced NBA “bubble” near Orlando, Fla., to finish off last season.

Because of the extended layoff, “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,” Stevens said.

In 57 games for the Cavaliers last season, Thompson shot 51.2 percent from the floor. He averaged 12 points and 10.1 rebounds in just over 30 minutes per game.

Boston has already said its high-scoring point guard, Kemba Walker, won’t be ready until mid-January at the earliest as he recovers from a knee injury.

The Celtics, who lost to the Miami Heat in the Eastern Conference Finals last season, open their season at home against Milwaukee.