Celtics coach Brad Stevens made the surprise announcement during his pregame news conference, just 90 minutes before tipoff. It was Green's first start of the season.
Isaiah Thomas also started after returning home to Tacoma, Washington, during the break between games to be with his family while he grieves the death of his sister.
"I think he's feeling better," Stevens said. "It's been a really hard week. He's going to have his ups and downs. I think going to Seattle was a good thing. ... I'm glad that he went. He's glad that he went."
Stevens had hinted at possible lineup changes this week and went with backup big man Tyler Zeller over Johnson to start the second half of Game 2. With Boston trailing 2-0 in the best-of-7 series, few expected Stevens to go with Green in a small-ball lineup.
Asked why he elected to go with Green, Stevens said, "Shooting and length and athleticism."
"We want to be able to spread them on offense and shoot the ball," Stevens said. "... On defense, I think if we're going to not play two bigs, we've got to be long and athletic on the perimeter, and we've got to go help each other rebound.
"So just because a guy starts doesn't mean he's going to play 40 minutes. But at the same time, I think when you have a chance to put a guy in that hasn't been playing as much, a good time to do it is with that first group."
Green said Stevens told him he'd be starting during Thursday's practice.
"Obviously, [Stevens] has a lot of trust in me," Green said. "My job is to do whatever to help this team be successful. Right now, [Game 3] is a must-win, by any means necessary. Just go out and play.
"Honestly, I feel like I play pretty good with IT [Thomas]. Feel like I've always played pretty good with him. I'm not going to do anything out of my character -- go out and try to focus on rebounding and defense and try to be the most athletic player on the court."