Kevin Durant fell, clutching his lower right leg. The consequences were so grave, Warriors teammates Stephen Curry and Andre Iguodala left the court to ease his path to the locker room. And an entire league went into shock.

There have been a number of catastrophic injuries over the years, drastically changing a team’s fortunes. Such names as Bill Walton, Greg Oden, Brandon Roy, Penny Hardaway, Bernard King, Yao Ming and Derrick Rose come to mind. But there hasn’t been anything like this. Today’s NBA is all about the summertime free-agent market, and everywhere you look, some carefully orchestrated plans just went up in smoke.

Only a few teams actually had a shot at signing Durant, in the event he chooses to leave the Warriors, but the trickle-down effect will be formidable, affecting all but a few franchises. And for purposes of this piece, let’s assume he decides to start a new basketball life in some other locale.

Start with Toronto’s Kawhi Leonard, the hottest free-agent story at the moment. He has no time to think about any of this — not with Game 6 of the Finals coming Thursday night at Oracle — but Durant’s departure weakens the Warriors, and the entire Western Conference, if he decides to head East.

Leonard has long fancied the idea of returning home to Southern California to play for the Clippers, who have an intriguing young roster and a smart, aggressive front office. All they need is a superstar, and Leonard would perfectly fit the bill. He easily could conjure up some championship dreams about an aging, fatigue-prone Warriors team finally giving up the throne.

If the Raptors win this series, bringing a first-ever title to the franchise, Leonard would have plenty of sensible reasons to stay. Blowing a 3-1 lead to the Warriors would crush Toronto’s festive mood, perhaps leaving Leonard believing he’s done all he can.

The Knicks have been the subject of Durant rumors for months, some sources claiming they’ve essentially wrapped up deals for Durant and Boston free-agent guard Kyrie Irving. The entire rumor mill is really just a pile of junk, with only a few odd kernels to be trusted, but now that Durant faces the prospect of missing an entire season, the Knicks need to look elsewhere.

(Always good for a tasteless back-page headline, the New York Daily News went with this Tuesday: “Knicks lose Game 5: Durant goes down with Achilles injury, ruining EVERYTHING as Warriors avoid elimination.”)

Worse yet for the Knicks, the crosstown Brooklyn Nets have privately expressed confidence about acquiring Irving. There’s also the looming presence of New Orleans’ Anthony Davis, who has made it clear that he wants to be traded. With Durant bound for the shadows, Davis can be a franchise-changer for several teams, with the Lakers and Celtics most likely to present the best offers.

Meanwhile, the free-agent hierarchy has undergone a major shakeup. Once Leonard makes his call, such names as Jimmy Butler, Kemba Walker, Tobias Harris, D’Angelo Russell and Khris Middleton will be more coveted than ever, with DeMarcus Cousins also in that mix and Washington’s Bradley Beal potentially available in trade.

What will the Houston Rockets have in mind? They’ve got their answer on Durant as far as next season, and that reported shakeup — perhaps involving expensive guard Chris Paul — might be put on hold. The idea of “running it back” already had gained credence with the recent reports that head coach Mike D’Antoni, believed to be heading into a lame-duck season with his contract expiring, is now back in negotiations for an extension.

Here’s the extent of Durant’s value and reputation: Even with his career in doubt, and next season probably lost, ESPN reported that three unnamed teams still would be willing to offer the maximum deal — four years, $141 million — to which he’s entitled if he leaves Golden State.

As for the Lakers, it seemed extremely unlikely that Durant had any interest. With the front office in chaos (too many voices, rampant distrust) and James approaching the end of his career, why would any of the other top free agents consider that team, especially with the Clippers on the rise? The Lakers have to push hard for a Davis deal — a package of Brandon Ingram, Kyle Kuzma, Lonzo Ball and the No. 4 overall draft pick would be hard to reject — or face another season of troubling irrelevance.

What was lost in Toronto on Monday night? More than anyone can accurately forecast. The Warriors were sent reeling into a state of depression, with tears all around. Durant’s career is in doubt, considering how many players couldn’t fully recover from an Achilles tear, which is what the Warriors are fearing it is. Such is the extent of the man’s greatness that he still might receive a max-contract deal from several teams.

In truth, though, nobody really knows anything. It’s stunning to realize that the clouds of uncertainty are about to blanket an entire league.

Bruce Jenkins is a San Francisco Chronicle columnist. Email: bjenkins@sfchronicle.com Twitter: @Bruce_Jenkins1