Per ESPN's Marc J. Spears, Bird received a two-way contract from the Celtics on Tuesday.
The NBA introduced two-way contracts for the first time this summer. It's part of a deal with the NBA Gatorade League that allows rosters to expand to 17 players, compared to the normal 15, and players on two-way contracts can spend up to 45 days on an NBA roster while earning a pro-rated league salary.
Boston selected Bird 56th overall in the 2017 NBA draft. He was one of the nation's best recruits coming out of high school in 2013, with Scout.com rating him as a 5-star player and the No. 13 player overall.
In four years at Cal, Bird averaged 10.8 points and 3.3 rebounds per game. He also shot 37 percent from three-point range.
The Celtics have quality depth in the backcourt with Kyrie Irving and Jaylen Brown locked in as their two starters. Marcus Smart, Terry Rozier and Shane Larkin will be getting most of the minutes off the bench, with Bird eventually coming in to offer head coach Brad Stevens another shooting option for his rotation.