BOSTON -- The NBA calendar is on the cusp of flipping into the postseason, a time when all the hard work put into building towards something special in the playoffs, begins to manifest itself. Leading the charge for Boston into the playoffs this season will be a healthy Kyrie Irving. But when it comes to progression in the postseason, one-man shows won’t cut it. The best teams have a pecking order of difference-makers, an order established by a season’s worth of opportunities by which those players either make good in those moments or melt under the pressure.

Which brings us back to the Celtics, who have a clear and undeniable top ace on the roster. That’s Irving. But who’s number 2? “Depends on the day of the week,” one NBA scout texted NBC Sports Boston. “That’s what makes them such a hard team to figure out and because of that, a tricky team to game plan against.”

As Boston gears up for these final 10 regular-season games, figuring out who the Celtics can turn to besides Irving heading into the playoffs continues to be one of the mysteries surrounding this club. 

Second-year wing Jayson Tatum was considered by most the logical sidekick to Irving this season. He was a breakout performer in the playoffs for Boston last year, averaging a team-best 18.5 points per game.

Tatum didn’t make the kind of jump many anticipated he would this season, but the 21-year-old is still averaging a respectable 16.0 points per game and 6.2 rebounds, which are both second on the team to Irving and Al Horford, respectively. And his plus/minus this season is +4.4, second only to Irving (+5.3).

But there are legit questions as to whether he is the next-best-thing on this roster, besides Irving especially down the stretch. Despite being the Celtics’ number two scorer this season, Tatum ranks fourth on the team in fourth quarter scoring (3.6 points per game).

And he’s not trending in the right direction when you consider his fourth quarter points per game has dipped to 2.8 since the All-Star break, which ranks seventh on the team despite averaging a team-high 8.1 minutes played in the fourth since the All-Star break.

So if it’s not Tatum, what about …

After Marcus Smart got tossed in the third quarter of Boston’s 118-115 loss at Philadelphia on Wednesday which proved costly to the team and Smart, there were many who believed had Smart not got ejected, his defense would have been the difference-maker to put Boston over the top.

But since the All-Star break, the numbers suggest that Smart is one of the most clutch players on this team offensively. Since the break, Smart has shot 54.2 percent in the fourth quarter which is tops among all Celtics to appear in at least 10 games since the break.

Yes, we love what Marcus Smart can do defensively down the stretch in games. And his playmaking has been on a steady rise from the moment the Celtics drafted him. But he has made noticeable strides as a shooter down the stretch as well.

The same can be said for Jaylen Brown, who may be the most overlooked Celtic player when talking about who should be Irving’s sidekick. People forget that Brown was the Celtics’ No. 2 scorer last season (14.5 points per game) to Irving when we all know Brown wasn’t getting No. 2 scoring option-type looks.

More than anything, Brown took advantage of the scoring opportunities he was given. No longer a starter, Brown has once again managed to maximize his opportunities in a reserve role. This season, he is averaging 12.9 points, which is tops among all Celtics backups and fifth on the team.

Brown is shooting 46.1 percent from the field this season, second only to Kyrie Irving (49.1 percent) among Celtics who are 6-7 or shorter. He’s also third on the team in free throw attempts (2.6) per game, which speaks to how he has been one of Boston’s more aggressive drivers to the basket off the dribble. But what really stands out about Brown is the fourth quarter, in which he averages a team-best 7.8 minutes per game. Of the Celtics players taking at least three field goal attempts in the fourth quarter per game this season, Brown is the only one shooting better than 50 percent from the field.

However, with lots of minutes on the floor comes an increased number of opportunities to make mistakes. And for all that Brown does well for Boston, he has shown himself prone to making the fourth quarter miscue, whether it’s driving the ball into a wall of defenders or making an errant pass which has collectively added up to Brown having a team-high 28 turnovers in the fourth quarter this season.

The player that the Celtics are most likely to lean on as the No. 2 guy to Irving is Al Horford. Nearing the end of his 12th NBA season, Horford has been a player whose statistics seldom speak to the level of impact he makes on a team, and that in itself makes him an unusual No. 2 option.

He ranks among the top four Celtics in a number of offensive categories, which includes a team-best 6.7 rebounds per game. But what really stands out about Horford is how he has performed in the fourth quarter of games since the All-Star break.

Since the break, Horford has averaged 7.8 minutes played, which ranks third on the team. During that time, he’s shooting a team-best 44.4 percent from 3-point range in addition to averaging 1.3 assists in the fourth, which is also tops among all Celtics players.

And Horford’s plus/minus of +1.8 in the fourth quarter of games since the break ranks second on the team among players with at least 10 games played over that stretch.

When you talk about the ups and downs of the Boston Celtics, that is an apt description of what this season has been like for Gordon Hayward. And while he has certainly taken his share of criticism, there’s no denying he has steadily improved throughout the season to the point where him having a big night scoring the ball isn’t all that surprising anymore.

But is he ready to be that No. 2 option for the Celtics? Probably not.

However, that doesn’t mean there won’t be nights in the playoffs when Hayward can be as impactful as Irving. We saw in Boston’s upset win at Golden State how big a difference-maker Hayward can be, a game in which Hayward dropped a 30-point bomb off the bench in helping lead the Celtics to an upset win. And the following night, he took his jacuzzi-hot scoring from the Bay and delivered what may have been a crowning moment this season for him when he drained a game-winning shot with just two seconds to play that would indeed be the difference in the Celtics’ win over Sacramento.

Indeed, Hayward has been a different — make that better — player for the Celtics since the All-Star break. He’ll be the first to tell you that he’s had a few clunkers mixed in, but his overall play has been noticeably better.

Since the break, he’s shooting 51.4 percent from the field which is second on the team in that span. His 3.2 assists in that span is fourth on the team but leads all Boston reserves. And maybe the most telling stat about his impact? When he’s on the floor, his plus/minus numbers since returning from the break stands at a team-best +3.9.

It wasn’t that long ago that the idea of Marcus Morris being this team’s No. 2 option was a reasonable one. But after being arguably the most consistent player on this team for the first half of the season, Morris has struggled to maintain that level of play since then.

His stats on the season still rank him among the better-playing Celtics. But there is no getting around how far his game has fallen off since returning from the All-Star break.

There was a point in the season when Morris was flirting with joining one of the more exclusive shot-making clubs in the NBA — the 50 percent shooting from field; 40 percent or better from 3-point range and 90-plus percent on free throws. For the season, Morris’ 14.2 points per game ranks third on the team. He’s still shooting a respectable 45.9 percent from the field and 38.2 percent on 3’s while making 83.6 percent of his free throws.

But those numbers are noticeably down, primarily because of how much Morris has struggled knocking down shots since the All-Star break. While his scoring hasn’t taken a major dip (12.7 compared to 14.5 before the break), his post All-Star break shooting numbers are down in a major way.

From the field, he’s making just 39.8 percent of his shots — worst among Celtics starters and players who log at least 25 minutes per game. And his 3-point shooting is down to just 28.6 percent, too.

But Morris plays with an edge about him that has been critical to Boston’s success this season. That helps, but they need more than that from whoever fills the void as the team’s No. 2 option.

No one questions the potential of Terry Rozier to be a high-impact difference-maker in this league. But his inconsistent play has made it unlikely that he can be the team’s No. 2 option after Irving, which in turn has been one of the many factors contributing to Boston not having the kind of success many anticipated from this group this season.

The maker of big plays in the playoffs last year, Rozier, much like Jayson Tatum, has not been able to get the kind of high springboard effect many expected from that success.

But having already experienced playoff success, one can’t count Rozier out from having a strong postseason next month regardless of who emerges, if at all, to become the team’s second option to Irving.