BOSTON — Thirty minutes after the final horn in the first game in which both Jaylen Brown and Jayson Tatum scored 30+ points apiece, Brown stepped in front of reporters inside the Boston locker room and offered something straight out of the Kaizen-soaked gospel of coach Brad Stevens.

"We still have our best days in front of us," said Brown, who followed up a monster 30-point performance on the Christmas Day stage in Toronto by scoring a career-high 34 points in a 129-117 triumph over the visiting Cleveland Cavaliers.

Brown wasn’t being modest. It’s fair to wonder if there’s another level these Celtics can go to.

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Boston still hasn't had its top players available together for much longer than a cup of coffee this season. Their potential best 5-man lineup has logged a whopping five minutes together this season. And they might just have their top 8 intact on Saturday night if Marcus Smart is able to suit up in a rematch with the Raptors.

What the Celtics have done to this point is undoubtedly encouraging. Despite all the injuries they weathered, Boston sits second in the Eastern Conference at 22-7 (a glitzy .759 winning percentage). The Celtics are fourth in the NBA in offensive rating (112.3), third in defensive rating (103.9), and now second in net rating (plus-8.3) behind only the East-leading Bucks.

FiveThirtyEight pegs the Celtics for 58 wins, the same total as the West-leading Los Angeles Lakers. And here’s the thing: We don’t even know what the ceiling is for these Celtics yet.

That’s wild considering all that’s gone right for Boston, with the play of Brown and Tatum at the top of that list. Over Boston’s last five games, Brown is averaging a team-best 26.4 points per game while shooting a ridiculous 60.3 percent from the floor and 47.2 percent beyond the 3-point arc. Tatum is close behind at 26 points per game while shooting 47.7 percent overall and 35.9 percent beyond the arc.

Brown scorched Cleveland not only with six makes at the rim (on eight attempts) but added five 3-pointers. Tatum played at times like he challenged himself to make nothing but highlight-worthy step-back jumpers and chipped in five triples of his own as well.

Watching it all unfold, Cavaliers coach John Beilein sounded a bit covetous.

“Every time I watch Boston, I’m very envious of what they have out there and remind myself that they were much like us five years ago,” said Beilein. “Watching Jaylen Brown hit those five 3s is just like something nobody would have dreamed a couple of years ago.”

Well, Brown probably dreamed it. Asked if his confidence has grown with his recent play, Brown responded, "The same confidence I've always had." Tatum played with a swagger well beyond his 19 years as a rookie and Stevens noted Friday how intrigued the team was by the mere potential of the Brown/Tatum combo as early as training camp before the 2017-18 season.

Not only are Tatum and Brown rolling, but the Celtics look more formidable with a healthy Hayward on the court. His ball-handling and playmaking helps create opportunities for everyone and Kemba Walker is always lurking ready to shoulder the scoring load on the nights everyone else is cold.

There are undoubtedly question marks, especially just how well Boston’s frontcourt can hold up against the likes of Joel Embiid and Giannis Antetokounmpo in a potential seven-game series. But Enes Kanter stumped for Boston’s bigs after his latest encouraging performance.

Now the Celtics will get back Smart, who has missed most of December, first when an oblique injury kept him off the court a bit at the start of the month then a freak eye infection took him out of commission for the past two weeks. Smart, who had been upgraded to questionable Friday before being ruled out, seemed likely to play Saturday against the Raptors.

Stevens admitted Smart would probably be on a minutes restriction, but even a tiny glimpse of how this team looks with most of its toys on the court might offer a better idea of where exactly this team sits among the crowded top of the East.

"I’m not worried about us reintegrating guys, not worried about how we play in small spurts. I’m worried about how we prepare and approach each game and then, ultimately, what our best version of ourselves looks like — and clearly Marcus Smart’s a part of that,” said Stevens. "We’re going to throw him in there if he’s able to play [Saturday] and let him play.”

With Smart back, the Celtics have their preferred starting 5 in Walker, Tatum, Brown, Hayward, and Daniel Theis — a lineup that, among 5-man units with at least 87 minutes played this season, ranks 1st in the NBA with a net rating of plus-24.6. It’s only a nine-game sample but it’s still a couple spots ahead of Milwaukee’s top 5-man group (Lopez/Matthews/Bledsoe/Middleton/Antetokounmpo at plus-19.5 in 117 minutes).

Now, Stevens can bring Kanter and Smart off the bench, easing the load on rookies like Romeo Langford and Grant Williams (though both have been excellent lately) and can mix-and-match his talent to best counter an opponent. Stevens can also finally experiment more with the super-small closing lineup with Smart replacing Theis in that starting group and determine if it’s a sustainable mix.

The Celtics have already proven themselves to be very good. Their perimeter talent distinguishes themselves from many of their rivals. But the schedule is a bear the next month-plus, with no more than a day off between games straight through to the All-Star break.

Boston is going to need good health to keep the good vibes rolling. And tough second nights of back-to-backs loom in Philadelphia and Milwaukee, which could help us better gauge where they stand.

Nights like breezy wins against teams like the Cavs are nice. Boston needs to figure out exactly what it’s capable of against the best in the league. The Celtics are hopeful that Brown is right and that their best days truly are in front of them.

Don't miss NBC Sports Boston's coverage of Raptors-Celtics, which tips off Saturday at 6 p.m. with Celtics Pregame Live, and then Mike and Tommy have the call at 7 p.m. You can also stream the game on the MyTeams App.