Many people were thinking this year would be a piece of cake for Brad Stevens as he reunited Kyrie Irving and Gordon Hayward with a roster that had made it to Game 7 of the Eastern Conference finals.

Doc Rivers was not among those people.

The Clipper (and former Celtic) coach on this cold Saturday morning was sitting on the scorer’s table across the court from a Reggie Lewis banner in the old gym where Northeastern used to play its games. He was talking about the Celtic pride reboot he and Danny Ainge had presided over when the latter hired him as coach in 2004. Then he looked to the present edition of the Shamrock AC.

“They have great culture,” Rivers said before preparing his team to meet the Celts in the evening. “It started back then, and it’s maintained. I mean, Brad does a great job with that. And now they have so many players, and that’s hard.”

With that, Doc dug into the issue that’s perhaps been most responsible for the 2018-19 Celtics not meeting even their own expectations — yet.

“People think that’s easy. Like, it’s hard,” Rivers said. “That is hard. I remember Coach K (Mike Krzyzewski) when I was at Duke watching (son) Austin play, he always talked about the separators. He said, ‘You know, what we really want is him to separate from everyone else, so we know. And then the next guy to separate from everyone else.’

“But when you have eight of those guys … You know, Kyrie obviously is the separator. But then everyone else in what line, and then you’ve got to get them to buy into that, and that’s hard, man. That’s hard. The fan thinks, well, they should all do it because they all want to win a title. They do want to win a title, but each guy thinks doing it the way he’s doing it is the reason you’re going to win a title. So it’s not as easy as it sounds. It’s very hard.”

Where Irving had to walk back some of his comments critical of the Celts’ younger players, Rivers laid out the issue in a basic, non-specific form.

“You’ve got to get them to play their roles, and young guys and roles don’t match,” he said. “Every young guy that comes in was probably an All-American, was probably the guy at his college. We told Al (Jefferson) he was the offensive player, and we told Perk (Kendrick Perkins) he was the defensive player, and Perk was like, ‘What the hell are you talking about? I scored 30 a game in high school.’ And we were like, yes, and now you’re a picker and you’re a roller. And it took Perk a while, I would say four years, to really like accept that role, and that’s hard.

“Everybody has All-Star dreams on their mind, where, as a coach, you only have winning. That’s your only goal. So you hope they make all that, but you really don’t care. You want them to learn how to win. And so I think Brad’s doing a great job. He’s teaching them how to win, and all the other stuff comes. But you’ve got to convince them that winning first, then everything else, you get. You don’t make All-Star first and everything else, you get. It’s usually winning first. That’s the order it should be in.”

Rivers is fully cognizant of the human nature at play here, too.

“They’re trying to establish themselves,” he said. “They’re trying to make who they are. I remember Perk and Al and (Rajon) Rondo, early on I used to think they played harder against each other in practice. And then once the minutes were established in the game, then they were cool. Like, no, that’s the opponent. We can’t be our own opponent. But it is not an easy task.”

The Celtics got some clear separation on their roster when Ainge traded for Kevin Garnett and Ray Allen in the 2007 offseason. But there was still the matter of getting those stars and Paul Pierce to harmonize. But with each having already gained the individual honors, they understood the larger picture.

“That could have been hard, but from the day they walked in…,” Rivers said. “Now, it was up to them to buy in, and they were at the point in their careers — and it’s where like Gordon (Hayward) is and it’s where Kyrie is right now — they’re at winning time. Everybody else is still at winning time, but they have other things they want to establish.”

Looking back at those ready to sacrifice, Rivers said, “(Al) Horford, he may be the poster child of it. He may be the poster child of ‘team.’ He’s on another level.”

The Celtics were stuck largely in the past when Ainge, a year into the job as head of basketball operations, brought in Rivers.

“When I took the job here, the first thing Danny and I talked about is how could we build a different culture than what was established,” Doc said. “And it was tough, because you were coming to a veteran franchise that had won, but had won years ago. We both felt like you can’t live on that. You know, every answer can’t be ‘that’s what we’ve always done.’

“When I went to the Clippers, we had an established group already, but the culture sucked. And that was harder, because the problem is you had a good team, but it was not a great culture. Now we’re trying to do it the way we want to do it.”

Ainge and the new Celtic ownership that hired him have built on the club’s storied tradition. Now, according to Rivers, it’s up to the present population to figure out how to write its own chapter.