The season, and sometimes a career, can end in a flash.

That thought was on the mind of Jeremy Lin last Wednesday in Washington when the Nets point guard ruptured his right patellar tendon. He not only was finished one game into the season, but when Lin shouted to the sideline, “I’m done,” he was pointing to something more final.

Gordon Hayward had gone down a night earlier, fracturing his left tibia and dislocating the ankle in what, too, is most likely a season-ending injury. This didn’t have the chilling finality of Lin’s shout. Hayward is expected to make a full recovery.

But if the Celtics are to maintain their position near the top of the Eastern Conference, Brad Stevens will face perhaps the biggest coaching challenge of his NBA career.

The Celtics are very much back to where they were last season with Isaiah Thomas as a singularly focused scorer — one who opponents could increasingly lock onto once the playoffs began.

Except that now Kyrie Irving will need support from some unproven parts of the lineup, and in particular 20-year-old Jaylen Brown and 19-year-old rookie Jayson Tatum. The latter has even more exciting upside than the former in terms of what he can become offensively.

But without Hayward’s playmaking and especially his 3-point shooting, the offensive potential of this lineup will have to be rethought.

But don’t say the Celtics are going back to square one, even this early in the season.

“Oh, no. We don’t got time for square one,” Irving said, almost wincing at the question. “I’ll tell you that right now. We don’t have time to go back to square one.

“It’s time to just figure it out moment to moment. And we’ll be all right. I’ve had the unique opportunity to be in some situations that tested your character. And this is one of those times. And, like I said, we just continue to be there for one another, look each other in the eye and be honest, make sure that our preparation is perfect, and going out there, and we have fun, and we’re playing the Boston Celtics way. And we’ll be fine.”

Irving was referring to the part of his past that’s been forgotten. Prior to LeBron James’ return to Cleveland for the 2014-15 season, the Irving-led Cavs had won an average of 26 games per season.

He knows about rebuilding and trying to find a workable combination. He’s been in a far worse situation than the one the Celtics now face in the wake of Hayward’s injury.

“As you can see, it’s not an ideal situation,” Irving said. “So as cliche as it is, everybody’s going to probably say that’s life. But it is, man. (Expletive) happens. And excuse my language for everybody here, but it does. The individual that we have in Gordon, he’s very special. And like I said before, he’s going to fight like hell to get back on the floor. I know he’s already — there’s a fire burning inside of him. We see the amount of support that guy got. That should tell you the type of person, and who he is, and what he represents — not only to the Boston Celtics but to the rest of the league. So we want him to get back healthy but we also understand that his health comes first. And he’s going to do everything possible to get back on the floor. And it’s our job to lift him up.”

Hayward admittedly would have needed more time of his own to settle into what the Celtics are trying to do. Now without him, though, there’s an appropriate amount of early confusion.

As Terry Rozier pointed out following last week’s loss to the Bucks, the Celtics are still at a stage where players are on several different pages.

“We just got a lot of guys that can play multiple positions, so when we call out the plays, some guys just run to the spots that they can play, and it just kind of messes us up,” Rozier said. “But like I said, we just gotta get it right. We’re professionals, we gotta just play off one another.

“(Wednesday) was the first game without (Hayward), so it’s gonna take time. The rotations are gonna be a little different, and I’m pretty sure that’s what Brad’s good at. He’ll make it work, we just have to worry about controlling what we can control and playing hard.”

Irving laughed when asked about Rozier’s lament about running the plays correctly.

“(What) we need in our preparation comes way before the games,” he said. “It’s going to come in waves and you see out there, we got (Abdel) Nader out there now, we got Shane (Larkin) out there, and to be honest with you, we haven’t even, in the first group, practiced with those guys. Now they’re out there playing big-time minutes and we need to utilize them and they gotta be in spots and know their spots 1 through 5. The reliance comes on the vets on this team and it trickles all the way down to the 15th guy. We gotta remain confident, unwavering in our approach, and we’ll be fine.”

This week’s C’s timeline

Tuesday vs. New York, 7:30 p.m. — The first week of the schedule hasn’t been an easy one, as opening losses to Cleveland and Milwaukee underscored last week. The Knicks, projected to spend the season in the Atlantic Division basement, offer a good opportunity to start working out some of these open problems. Mark this as a potential debut date for Marcus Morris.

Thursday at Milwaukee, 8 p.m. — The Celtics were overmatched by this team last week in the Garden, with Giannis Antetokounmpo able to score at will against anyone thrown in his way. Brown, whose defense is being stressed by Stevens this season, gets a second early chance against the Bucks star.

Saturday at Miami, 8 p.m. — Though Gordon Hayward’s absence ultimately will limit what the Celtics can do against certain teams, this is one game — thanks to the presence of Hassan Whiteside — that could urge Stevens to go with his big lineup. The Aron Baynes/Morris combination might be interesting here.