John Tavares. Erik Karlsson. John Moore?

The Atlantic Division has an invisible force field after the top three teams: the Tampa Bay Lightning, Toronto Maple Leafs and Boston Bruins. One of those teams has made an enormous roster upgrade this summer. The other is reportedly attempting to do the same. And one team counts a rugged bottom-pair blueliner as its biggest acquisition.

The Toronto Maple Leafs are far from a perfect team. They were the most porous of any squad to make the playoffs last season, allowing a whopping 33.9 shots per game in the regular season. But even if he’s giving the Leafs more of what they already have, John Tavares makes them that much tougher to stop. The Leafs are a strong bet to lead the NHL in goals after ranking fourth last season, and with their puck control up the middle from Tavares, Auston Matthews and Nazem Kadri, it stands to reason they’ll allow fewer shot attempts this coming season. Tavares will help there when he’s on the ice, and should the subtraction of old-school Roman Polak from the defense corps. So a team that led the Boston Bruins in the third period of Game 7 in Round 1 of the playoffs adds arguably one of the top 10 players in the sport.

Reports vary on how close the transaction is to happening, but the Tampa Bay Lightning are zeroing in on acquiring blueliner Erik Karlsson in a blockbuster deal with the Ottawa Senators. The Bolts were even better than the Leafs last season and rank up there with the Winnipeg Jets for the NHL’s best collection of all-around star power at every position. Nikita Kucherov, Steven Stamkos and Brayden Point up front. Victor Hedman, Ryan McDonagh and potentially Karlsson on defense. Vezina Trophy finalist Andrei Vasilevskiy in net. And this collection of talent was good enough to outclass the Bruins in five games in Round 2 of the playoffs – before acquiring Karlsson. Even if the Karlsson trade falls through, its stands to reason the Bruins need to improve their roster to catch up to the team that disposed of them rather easily.

And the Bruins, a Cup contender last year and a projected one this year, have fallen behind their two biggest division rivals in the off-season arms race. Trade-deadline rental Rick Nash draws out of their lineup now that he’s become a UFA and has stepped away from hockey to contemplate his future. The other Nash, Riley, was a crucial contributor for the Bruins last year, making an impact as a checking-line center and filling in admirably on the first line between Brad Marchand and David Pastrnak when Patrice Bergeron was hurt. Riley Nash now plays for the Columbus Blue Jackets, having signed as a UFA. The Bruins were talked up as suitors for left wingers Ilya Kovalchuk and Mike Hoffman but wound up with neither. They were one of the six teams allowed to pitch Tavares in person, too, but lost out.

Not only has Boston’s stiffest competition upgraded significantly, but the Bruins on paper look worse. John Moore and Jaroslav Halak do not equal the two Nashes and Anton Khudobin. Should Bruin fans worry? Is GM Don Sweeney losing the 2018 off-season?

There are two ways to view the situation. One is, yes, Boston’s in trouble. Captain Zdeno Chara enjoyed a remarkably resurgent season as a shutdown defenseman and has been the best mentor rising young star Charlie McAvoy could ever want, but Chara will be 42 by the 2019 playoffs. He’s a no-doubt Hall of Famer, but his game will fall of a cliff one of these years. Just ask Jaromir Jagr. Up front, Bergeron, David Backes and David Krejci will be 33, 34 and 32 on opening night. Bergeron remains the driving force of hockey’s most dominant line, and that likely won’t change this coming season, but it would only be reasonable to expect decline eventually. It’s the science of aging. Bergeron missed 18 games this past season and underwent off-season groin surgery. The Leafs and Bolts’ best centers are much younger than Bergeron and Krejci.

As Boston’s top vets age out, that will put pressure on the team’s youth crop, and that’s actually where the news is good. The Bruins have a promising next generation, of which some members are contributing already and some are about to start. Pastrnak was already becoming a top-flight offensive player and may have broken through to true stardom in the playoffs. Power forward Jake DeBrusk had a nice rookie season. McAvoy looks like a future Norris Trophy winner. Ryan Donato immediately showcased tantalizing scoring touch after turning pro last year. Towering Brandon Carlo has legit shutdown ability when he’s healthy. Danton Heinen looked like a steal in his rookie year. And Jakob Forsbacka-Karlsson’s mature two-way game gives him a strong chance to make the NHL this year and fill Riley Nash’s role, but with more skill.

So the Bruins still have plenty to be excited about, especially with Marchand, blueliner Torey Krug and goalie Tuukka Rask still in their primes. This is still one of the best two or three possession teams in the NHL. They’re mentioned in the same breath as Toronto and Tampa for a reason. The Bruins might still be a 100-point team. But is that going to cut it considering the Chara-Bergeron generation is running out of chances at a first Cup since 2011? Fair or not, the Tavares and rumored Karlsson moves put heat on the Bruins and GM Sweeney to answer

The free agent market has dried up and was relatively barren to begin with this summer, so if Sweeney wants to make a move, it’ll have to be via the trade route. Keep an eye on speedy types such as Jeff Skinner and Jason Zucker or, if they want to make their forecheck heavier, Wayne Simmonds. Maybe a Milan Lucic reunion makes sense depending on whether the Oilers eat some salary. The Bruins do have a surplus to work with, as Moore piles onto a defense group that already has far more NHL-caliber blueliners than starting roster spots. The question, of course, is who goes in a deal for a forward. Is it worth moving Krug, who would bring the best return? Or are the Bruins better off keeping Krug, an excellent puck-mover, and just hoping the young forwards continue ascending?

These are tough questions, and they become much more important for Sweeney to answer if Tampa secures its monster upgrade to match Toronto’s.