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When Danny Ainge overhauled the Celtics’ bench in the offseason — adding Abdel Nader from the G-League, drafting Semi Ojeleye and Jabari Bird, pulling Shane Larkin out of Spain, signing Daniel Theis from Germany, and plucking Aron Baynes from the Detroit Pistons — they were simply supposed to be complements.

Bird was headed for the G-League. Nader would play sparingly, same with second-round pick Ojeleye, who last year was playing in the American Athletic Conference. Larkin was supposed to be the third point guard. Theis was a relative unknown.

The severe and disheartening injury to Gordon Hayward has forced coach Brad Stevens to go deep into his bench over the first week of the season, with every player on the roster not only playing, but playing meaningful minutes.

Take Bird for instance. He was at home in Vallejo, Calif., packing his stuff and preparing for the Maine Red Claws training camp when he got the call that he was to immediately report to Boston. On Friday against Philadelphia, he played 13 minutes, scoring 3 points in his NBA debut. The Celtics had projected him to play mostly in Maine, then perhaps join the big club when they needed depth. Well, they needed depth now.

“His defensive presence stood out,” Stevens said. “And I knew when he caught the ball he was going to let it fly. He doesn’t have much hesitation. He doesn’t have much fear. He’s just going to go out and play.”

The reality is Stevens doesn’t have many options besides going young. In order to add Hayward and trade for Kyrie Irving, the Celtics had to part ways with some veteran players. They knew they would be top-heavy in terms of a starting lineup but believe the trio of Irving, Al Horford, and Hayward would be good enough to compensate for an inexperienced bench.

Not even six minutes into the season, Hayward was felled with a dislocated left ankle and broken left tibia, leaving Stevens to scramble for viable lineup options.

Then Marcus Smart sprained his ankle in the loss to Milwaukee and missed Friday’s win over Philadelphia.

Marcus Morris, a much-needed veteran frontcourt player, may be a week from returning because of a sore knee. So it’s all about the youngsters, unknowns, upstarts, and overlooked players for now.

“Every one of them should think there’s a good chance that [they] get an opportunity to impact the game on Tuesday night,” Stevens said. “There’s not a guy on this roster who shouldn’t think that right now. It may or not be their night. But we’re in a situation where we need everybody.”

Irving is in his seventh season and realizes, despite being just 25, that he is a leader and mentor for the younger players. He has repeatedly said his young brethren need to be ready.

“I don’t like to get into the whole big brother, little brother thing, but having the year of experience, I think that caters to me having some knowledge that I can offer,” he said. “Philly was nothing. We’re going to see a lot more hostile environments going forward on the road, and we’re going to have to depend on one another.”

The past few days, the Celtics have had to essentially start over. Stevens has been using rag-tag lineups, depending on players he never envisioned playing so early, and then shifting his late-game offensive focus more toward Irving and Horford with Hayward out.

“I almost felt like in the last three days we’ve been early in training camp in a lot of ways,” Stevens said. “We talked today. Everybody talks about young players having to learn by going through experience. Let’s beat the age thing. Let’s not talk about the age thing. Let’s talk about how we can be better and what we can control and how we can learn and grow every day. ”

Said president of basketball operations Danny Ainge: “There’s going to be some adjustments. Brad has a very difficult job ahead of him, and so do our players. [Kyrie] came here with a great deal of energy, enthusiasm, and confidence in Gordon and Al. It will be easy for them, but it will take time for all the players to gain the confidence of the coaching staff and one another.”

Bird is benefiting from the Celtics’ search for answers. Stevens has been fond of his skills since he was drafted, and now he and the crew of younger players, minor free agents, and European imports will be a critical part of the Celtics’ success and their quest to maintain their lofty goals of winning the Eastern Conference.

“You have to prepare for anything at any given time,” Bird said. “So I just try to stay locked in. I felt bad for Gordon and his whole situation. I got the call and quickly told my dad I gotta go to Boston. We’ll see where I go from there.”

. . .

The NBA has fined Irving $25,000 for responding to a fan with inappropriate language.

A video went viral late Friday night showing Irving shouting an expletive at a fan in Philadelphia who had yelled to him asking for the whereabouts of former teammate LeBron James.

Irving on Saturday said he did not regret his choice of words.

“Hell no,” the Celtics guard said. “Man enough to record it on video, then that’s on him. I’m glad he got his [social media username] out there and then kind of five seconds of fame and it going viral. That’s the social media platform we live on. I take full responsibility for what I said, and excuse the kids at home, and you move on.”