Rajon Rondo and the Celtics haven’t had much to smile about lately — they lost 11 of their last 21 regular season games — but nobody in the Eastern Conference is taking them lightly. (Mike Ehrmann/Getty Images)

Considering the ascending powers, stitched-together lineups of stars and question marks in the Eastern Conference, it is difficult to say who will rise to the top as the NBA postseason begins this weekend. But there can be little debate about which team elicits the most discomfort and unease.

In a matter of weeks, the Boston Celtics plunged from the East’s top seed to the third. They lost 11 of their last 21 regular season games. Their March trade of big man Kendrick Perkins to Oklahoma City proved an unmitigated disaster.

“The frustration is high right now on our team,” Boston Coach Doc Rivers said last weekend.

Yet despite its myriad struggles, Boston threatens to bring about the biggest headaches for everybody else in the East. They are the road over which no one wants to travel. The Celtics’ veteran stars are figured either to cook up another postseason resurrection — like last season when they took the Los Angeles Lakers to a Game 7 in the NBA Finals after entering the playoffs as a fourth seed — or wear down and effectively take out other legitimate candidates with their brutal, defensive style.

“Everybody should have learned from last year not to downplay these guys,” Miami Heat forward Chris Bosh said. “They’re still a good team.”

They just haven’t looked like it lately. Boston capped its latest ugly skid into the playoffs with Monday’s narrow loss to the Washington Wizards and a thorough manhandling by the Heat the day before, before getting a victory over the New York Knicks, their first-round playoff opponent, in a meaningless season finale in which both teams’ stars rested.

In the Western Conference, the defending champion Lakers (No. 2 in the West) ended the regular season by losing five in a row before winning their last two games. But despite some angst in Los Angeles, the Lakers still have Kobe Bryant, a stellar supporting cast and the experience to make a strong run — starting Sunday against the New Orleans Hornets, who lost David West to a left-knee injury late in the season.

San Antonio, the No. 1 seed in the West, also tapered off at the end of the season, losing six of its last 10 contests — and leading scorer Manu Ginobili to a sprained right elbow. He is officially listed as doubtful for Game 1 against Memphis on Sunday, but the Spurs are still favored to top the No. 8 Grizzlies.

Two of the most intriguing teams in the West meet in the first round: Oklahoma City and Denver. The Nuggets (No.5) went 18-7 after trading Carmelo Anthony, while the fourth-seeded Thunder gave the Lakers a tough first-round test last year and acquired the veteran Perkins to get tougher in the paint. The Dallas Mavericks (No. 3 seed) face a difficult first-round challenge against the Portland Trail Blazers, who are considered dangerous underdogs.

The Celtics’ series against New York, which opens in Boston on Sunday, promises great story lines and good theater, though surely not elegant basketball. (That might come in Saturday’s opening-round contests between the Chicago Bulls and Indiana Pacers or Heat and Philadelphia 76ers.) The signature moment of the Celtics-Heat contest in Miami last Sunday came when Jermaine O’Neal used a shoulder blow to prevent LeBron James from scoring on a fast break, a near-tackle that did nothing to determine the outcome of the game, but ignited shoves and angry words and referees’ whistles.

James and teammate Dwyane Wade had discussed the problem of playing the Celtics before Sunday’s tip-off, Wade said after the game, agreeing that the “one thing this team is good at is making you try too hard because of the dislike against them.”

There can be a lot not to like. Boston’s stars — Kevin Garnett, Paul Pierce, Ray Allen and Rajon Rondo — took their 50 regular season victories last year and resoundingly defeated the Cleveland Cavaliers (61 wins) and the Orlando Magic (59) in the first two rounds of the playoffs, using suffocating defense and unselfish play.

Once again, they threaten to get in everybody’s way. If Anthony, Amare Stoudemire and Chauncey Billups want to show they are more than a disjointed collection of stars for the New York Knicks, they will have to push aside the Celtics. If Miami expects to get to the Eastern Conference finals, it might face Boston in the conference semifinals. And if Boston mimics its determined playoff run from last year, the top-seeded Chicago Bulls will have to beat Boston, too.

“We will have to see,” ESPN NBA analyst Jeff Van Gundy said, “if Boston gets itself together.”

Chicago, in contrast, seems to be very much together, and Miami seems to have pulled itself together after a soap opera of a season. At first the Heat couldn’t beat anybody, then it couldn’t beat anybody good. Every few weeks or so, Coach Erik Spoelstra’s job seemed to be in jeopardy or at least, many suggested, it should have been. Distractions abounded. James’s mother even got arrested. But by season’s end, Miami’s star trio finally seemed in synch.

“Our teamwork is a lot better,” Bosh said. “Before we were always trying to make the big play individually. Now, we have more trust. We get more assists. We attack the paint more . . . . We make the defense move. We hunt down the open shot.”

Van Gundy liked what he saw of Miami last weekend. Though Chicago cruised to the Eastern Conference’s top seed, winning 24 of its last 27 games while defeating the Heat in each of three meetings, Van Gundy figures Miami’s best — namely James, Wade and Bosh — will get more mileage in the playoffs than Derrick Rose, Luol Deng and Carlos Boozer.

“Chicago has had an incredible regular season,” Van Gundy said. “They have the MVP [Rose] and coach of the year [Tom Thibodeau], and they’ve absolutely gotten the most out of their roster. But the most talented team? You put those three guys [from Miami] together and it will be hard to beat them in a series.”

Unless Boston wakes up.

Getting Shaquille O’Neal back from a calf injury that sidelined him at the end of the season would help as the team struggles to fill the void left by the trade of Perkins — a deal that mystified the Celtics and their fans. Without a big presence inside, Boston struggled defensively and offensively. The Celtics hit triple digits only five times in their last 20 games.

“We’ve been in this rut, not scoring points, the last five, 10, 15 games,” Allen said last weekend.

The Celtics have seemed positively annoyed by their stumbles but hardly frightened or concerned. Rivers said after Sunday’s loss his team would love to face the Heat again in the playoffs.

It might not be apparent, he said, but the Celtics are ready to make things difficult for everyone.

“The East is tough,” Rivers said. “I think it gets overlooked at times. It’s a brutal conference this year; I don’t think there are any gimmies in any round. . . .You have to be ready when the playoffs start. It should be fun.”