BOSTON -- The citizens of Boston could rest easy last night and sleep late this morning. The Celtics had preserved the city's honor, beating the Lakers by 15 points and sending the NBA finals back to L.A. No linen-suited, celebrity-toting, show-time, party-animal aliens would win an NBA championship on The Gahden's fabled parquet floor. That they'd have to do in La-La Land. Not here.

The Celtics didn't blow a 16-point lead this time. They got it up to 19 at the end of the third quarter and persevered through a Lakers mini-quake that cut it to eight with 5:58 left. They got great efforts out of all their starters -- all five scoring 20 or more; Danny Ainge nailing five 3s, including one at the end of the first half from a Rhode Island area code -- and sent their fans home secure in the knowledge that whatever else happens, the Celtics wrung every last drop from the fruit this season. They went a stirring seven with Milwaukee, a cardiac-arrest seven with Detroit, including the Bird Miracle in Game 5, and now they've pushed the Los Angeles Lakers sprint medley relay team back to the track again. "There was no way I was going to let us lose here," Kevin McHale said. "I've been here seven years and the fans have been very good to me. I wasn't going to let it end here with a bad taste in my mouth and their mouths." So what if the Lakers win it out there? So what if this was a reprieve, not a pardon? Boston fans have left the last home game happy. Their Celtics can be defeated, but they cannot lose.

Granted, where there's life, there's hope. But no team has ever come back from a 3-1 deficit in the final series. As Larry Bird said the other night, "Our chances of winning this thing are tough, and everybody knows it." That still holds. James Worthy and Byron Scott all but disappeared here -- not-ready-for-prime-time players in a half-court game. But the Lakers are a different team at home. And so, decidedly, are the Celtics a different team on the road. They were 36-1 here during the regular season, but 20-21 on the road. In the playoffs, they lost two of three in Milwaukee, and three of three in Detroit. Now, they are being asked to win two in a row in L.A. where they're 0-3 this season. And the first two games of this series they were thrashed by 13 and 19 points.

If the Celtics can keep the Lakers from running there as they have here, they can win. But the pattern of this series says it isn't likely. What is likely is that, for the 18th straight season, the NBA will crown a different champion. "There's always Sunday," Magic Johnson said confidently, referring to Game 6 in L.A.

The last repeat NBA champion was Boston in 1969, so long ago that Abdul-Jabbar was still known as Lew Alcindor. It was his rookie season, and Bill Russell's final. Since then it's been one sip of champagne and close the door on your way out. Baseball and football haven't had repeaters in a while either (1978 Yankees, 1979 Steelers), but it seems it shouldn't be as hard in basketball. Unlike baseball, the NBA lets every good team -- and a lot of bad ones -- into the playoffs. And unlike the NFL, it's not single elimination; one fluke game won't capsize you.

What makes the recent NBA history so odd is that the laws of probability alone should have been enough to ensure a repeat champion. In the 1980s, only four teams -- the Celtics, Lakers, Philadelphia 76ers and Houston Rockets -- have played in the finals. Boston and L.A. have been in 11 of 16 finals berths, and even with that limited a pool, there still hasn't been a repeat. Bob Ferry, general manager of the Bullets, says that has less to do with any psychological factor, such as the dreaded Fat Cat Syndrome, than with specific personnel changes. "Two things stand out: injuries and trades," Ferry said. "When you get to the level where you know you're a good team, you make specific moves to beat specific teams you know you're going to have to play. We did that. We knew we'd have to beat Philly, and we knew we had no one to guard Julius Erving. That's why we got Bobby Dandridge."

Looking at the list of nonrepeaters we can see examples of the conspiracy of injuries and personnel moves: Bill Russell retiring opened the door for the 1969 Knicks; a wizened Oscar Robertson joining the Bucks gave Milwaukee the generalship it needed in 1970; Bill Walton's injury doomed the Trail Blazers in 1978, the year the Bullets won; the entry of Magic Johnson and Larry Bird in 1980 completely upheaved the NBA; Moses Malone signing with the 76ers as a free agent unseated the Lakers in 1983; Boston's adding Walton in 1986 made an already great front line dominating.

People looking back at 1987 will see critical factors both in injury and personnel moves. The most important move the Lakers made was changing the team from inside and Kareem-oriented to hard-charging and Magic-oriented. Making Magic the focus helped Worthy, Scott, A.C. Green and Michael Cooper be more involved in the offense. For the first time in his career, Kareem averaged fewer than 20 points this season (17.5), while Magic rolled up his biggest numbers ever, 23.9, and the Lakers won 65 games. In the similar spirit of Boston's adding Walton, the Lakers' acquisition of Mychal Thompson was the tinkering that gave L.A. a comfort zone in front-court depth.

Ironically, the Celtics have been gravely minimized by Walton's injury. That, combined with Scott Wedman's injury and the tragic, shocking loss of Len Bias -- who'd have made the Celtics awesome to contemplate -- left Boston ill-equipped to defend its championship. In reality, Boston is approaching the slippery slope. Robert Parish will be 34 next season, Dennis Johnson 33. Even in the unlikely event both Walton and Wedman return, they'll be 35. The bench has proven itself as thin as a novella, and there wouldn't appear to be salvation in the draft. By next year at this time, one of the young suitors, most prominently Atlanta, figures to usurp Boston's perpetual dance in the finals, and find a familiar partner there, the Lakers.