Only one thing will make this season a success, and no, trading Nate Robinson doesn’t count. The Celtics will either win an NBA championship or enter the offseason as a failure, with an NBA lockout likely and no promise the Big Three will ever play like the Big Three again.

In retrospect, that’s why I responded so harshly to the Kendrick Perkins trade. With Perkins as the starter, we knew the Celtics could win a championship. They’d already won one, and as Doc Rivers has said, the starting five with Perkins was never beaten. They won one title, lost in the Eastern Conference semifinals when Kevin Garnett got hurt, and lost in Game 7 last season after Perkins went down in Game Six. With Perkins as a starter, the Celtics were 7-0 in playoff series; 7-0-1 if you count the Lakers series as a tie.

We had faith in Perk, because we knew everything he was about. He was going to scowl, throw elbows, notch a technical foul once every three or four games, limit the opposition’s best post player (unless said post player was Andrew Bogut, who Perk never quite figured out), hold his own on the glass, scowl some more, make a few short fadeaway jumpers over his left shoulder, take his sweet ass time while releasing layups, and bring a physicality rare in the NBA. With Perk, the Celtics intimidated other teams. With Perk, the Celtics’ starting five proved it could win a championship. Without him, well, questions needed to be answered.

Can Nenad Krstic keep the defense functioning at a similar level? Can Shaq ever return to health? Who becomes Boston’s enforcer when the playoffs come and another team lowers the boom with a hard foul? Can the Celtics still defend Dwight Howard one-on-one? Does the way Howard’s teammates have played make the previous question a moot point? Can the Celtics still punish Miami inside? Do they have enough to limit the Lakers if they see them again? Is Jermaine O’Neal even alive? Are the Bulls as good as they’ve looked during the regular season? Can the Celtics handle them?

Many of those questions, of course, existed even when Perkins still wore Celtics green. I understand that. Perk’s presence did not change the O’Neal brothers’ health. It did not change that the Bulls are now a legitimate threat. Hell, Perk was injured, and his questionable health only would have added more question marks. Still, there was something comforting about knowing the Celtics had been there before as a unit. That they’d won with Perk as their starting center. That they were still undefeated in the postseason with their starting five intact.

That same feeling of familiarity no longer exists. Even now, after last night’s dismantling of Milwaukee and a mostly-impressive start to the Krstic/Green era, the new-look Celtics have yet to play a contender. They haven’t played LA, Miami, Chicago, San Antonio, or even Orlando or Oklahoma City. We don’t know how the newcomers’ success will translate against the league’s best. We don’t know how they will respond to the playoffs, and, really, neither do they. Green has played in one playoff series during his career, and Krstic has never advanced past the second round.

In short, we don’t know nearly as much about Nenad Krstic or Jeff Green as we knew about Kendrick Perkins, nor do we know as much about the new-look Celtics as we did the old-look Celtics. That lack of knowledge leaves us with a hint of wariness, of course. But as we learn more about Green and Krstic, as we learn more about the new-look Celtics, the possibilities intrigue.

We know the post defense won’t be the same*, since Krstic allows better post position than Doc Rivers would like and too frequently seems a step late on rotations. But neither will the offense from the center spot, where Krstic is A) a better shooter than Perkins, B) better at moving without the ball, C) far less prone to turnovers, and D) actually able to catch-and-shoot before an hourglass expires. We expected the rebounding would decrease, but Krstic has quickly become a Serbian Dennis Rodman, and shows no signs of slowing down his suddenly-passable rebounding rate (though my Rodman comparison was certainly hyperbole). We expected Krstic and Green would take a long time to adjust to their new mates, but both seem to have learned their roles quite quickly.

*(In the “weird stat of the day which is probably due to small sample size,” Boston’s starting lineup has performed worse offensively with Krstic than it did with Perkins. And the defense? Wouldn’t you know it, a little better with Krstic than Perkins.)

Green, already, has teamed with the other newcomers (Arroyo et al) to transform Boston’s second unit. Believe me, I spent the first half of the season knowing — just knowing — Boston’s second unit would blow a lead, or kick away a tie game, or turn a small deficit into a considerably larger one. As Boston’s second unit continually blew games or made them far closer than they should have been, I spent far too much time cursing Nate “I love shooting pull-up three-pointers in transition, even though I hardly ever make them” Robinson, wondering why Marquis Daniels produced so little, and hoping Semih Erden would finally start turning his high basketball IQ and solid athleticism into points and rebounds. Now? I no longer expect a second unit blowup, because Green, Arroyo et al have instantly changed the second unit into a functional, capable bunch. My favorite contribution Green has made so far? That I no longer dread when Doc Rivers makes substitutions.

Kendrick Perkins sat in Oklahoma City’s locker room today, calling Pau Gasol soft and Phil Jackson arrogant, making clear for all to hear: the Los Angeles Lakers will not push the Oklahoma City Thunder around, not anymore, not on Perk’s watch. It was the latest sign of the toughness we Celtics fans came to expect from Perk, except now, he makes his claims on behalf of another team.

When the Celtics need that type of toughness this postseason, they’ll have to turn to someone else. Perkins won’t be there to lend an elbow and a scowl. He won’t be there to go toe-to-toe with the league’s biggest players. He won’t be there at all, but Nenad Krstic and Jeff Green will. They won’t provide the same things Perk did, and nobody would ask them to. But they help, in ways Perk couldn’t, in ways that keep the Celtics quite dangerous, in ways that keep the Celtics at (or at least very near) the forefront of any NBA championship discussion.

Perk’s gone, of course. But the Celtics’ new starting five is still undefeated in the playoffs, no?