Three games into the season, the Boston Celtics already have used three starting lineups.

They haven’t done that since the lockout-shortened 1998-99 season when Antoine Walker, Paul Pierce and Kenny Anderson were the only lineup constants over the first five games, according to the NBC Sports Boston stat guru, Dick Lipe.

Fourteen players, including Jabari Bird, who watched the opener on television at home in California, already have received playing time.

It was all hands on deck when the Celtics got their season going last week with three games in four nights.

They got things going without Marcus Morris, a potential starter who is out with a sore knee.

Then Gordon Hayward suffered a gruesome injury less than six minutes into the first game at Cleveland, putting him out for the season.

And Marcus Smart, who joined the starting lineup the next night for the home opener against the Milwaukee Bucks, had to sit out Friday night’s victory in Philadelphia with an ankle ailment. That led to Aron Baynes becoming a starter against the 76ers.

After three weeks of preparation, the Celtics found themselves changing on the fly in the first week of the season.

“There’s so much we’ve tweaked from the last two or three days,” said Coach Brad Stevens, “just because we’re changing a little bit of our emphasis, specifically our emphasis late game, some of the things we’re trying to do with regard to getting Kyrie (Irving) and Al (Horford) in better spots for them, getting some of our young wings into the post.

“We’re subtracting a little bit of what we put in at the start of the year, which now seems like a long time ago. I almost felt like we’re early in training camp again in a lot of ways.”

The Celtics, who are home Tuesday night against the New York Knicks before hitting the road for two games, were stunned by the Hayward injury.He had surgery on a dislocated left ankle Wednesday night and also has a fractured tibia.

Without Morris and Smart, the Celtics have gone deep into their bench with five rookies –Daniel Theis, Bird, Guerschon Yabusele, Abdel Nader and Semi Ojeleye – part of the second unit, and No. 3 pick Jayson Tatum starting all three games.

“Coach said it best in the locker room,” said Irving, who Sunday was fined $25,000 by the NBA for using inappropriate language to a fan in Philadelphia. “You don’t necessarily have to wait for experience. They always said experience is the best teacher but these young guys, they don’t have to wait for that opportunity now and wait for an impact.

“We need them to contribute. For me to see that and witness it, makes me nothing but proud and happy to have teammates that are ready to play.”