The option is part of the two-year, $11 million deal he signed with the team last summer.
Baynes is coming off a season in which he averaged 5.6 points and 4.7 rebounds in 16.1 minutes per game. He registered 18 starts and 51 appearances during the 2018-19 regular season.
While Baynes was used in a limited role this past season, he was a key part of the rotation as the Celtics came within one game of the 2018 NBA Finals without Kyrie Irving and Gordon Hayward. The 6'10", 260-pound big man averaged 6.0 points and 6.2 rebounds in 19 postseason appearances, including 12 starts.
As Charania noted, though, Baynes' value to the Celtics cannot be determined strictly by raw numbers:
After spending three years with the San Antonio Spurs and two with the Detroit Pistons, Baynes has settled in since he arrived in Boston in 2017. That comfort level reportedly played a role in his decision to remain with the team for another season.
"No-brainer," a source told Yahoo Sports' Keith Smith of Baynes' decision to exercise his player option. "Boston is the first place where he's felt truly at home in the NBA. They want him and he wants to be there. And $6 million bucks doesn't hurt either!"
Smith added that Baynes exercised his option with the understanding that he would not be traded this summer.