Joe Vardon of Cleveland.com reported Irving threatened to first sit out training camp and then undergo surgery that would keep him out most—if not all—of the season. The report says Irving needs a clean-up procedure on his right knee, in which he suffered a broken kneecap during the 2015 NBA Finals. That procedure will in all likelihood be done this offseason.
Irving requested a trade this summer in part due to his displeasure playing next to LeBron James and because the Cavaliers floated his name in talks trying to acquire Paul George. The Cavs eventually sent him to the Boston Celtics in exchange for Isaiah Thomas, Jae Crowder, Ante Zizic, the Brooklyn Nets' 2018 first-round pick and a second-round pick—a move that has backfired for Cleveland.
Thomas has missed most of the season due to a hip injury and has struggled since his return. Crowder has been a surprising bust, going from a very good two-way rotation player in Boston to being a net negative on both ends of the floor. The Nets have been surprisingly competent this season, making their potential top-five selection look like it could even be outside the top 10.
Irving, who has unsurprisingly put off that knee surgery, is in the midst of leading the Celtics to the No. 1 seed in the East. He's an outside MVP candidate who is shooting a career-best 47.7 percent and has improved defensively under Brad Stevens.
Threatening to sit out part (or all) of the 2017-18 season if the Cavs refused to trade him, in all likelihood, was a bluff. Irving is an in-his-prime star with a ton of endorsement money on the table. The Cavs haven't exactly been a sealed vault avoiding leaks; it wouldn't be hard to smear Irving if he actually went through with the threat.
That said, Irving being even willing to threaten sitting out games that count is evidence of how toxic the relationship became.