The All-Star, who was limited to 60 games, will undergo another operation on Saturday after a bacterial infection was found in his left knee.
They have been surviving despite playing shorthanded since opening night of the 2017-18 NBA season more than 24 weeks ago.
The Celtics found a way to overcome the shocking loss of All-Star forward Gordon Hayward less than six minutes into their first game, compiling the league’s fourth-best record.
They even managed to compensate for late-season injuries to Kyrie Irving, Marcus Smart and Daniel Theis, winning games after they underwent operations in March.
After bouncing up off the canvas all season long, however, the Celtics were hit with what should be a knockout punch on Thursday.
That is when they learned that Kyrie Irving’s season is over with another surgical procedure on his left knee scheduled for Saturday, two weeks after the All-Star guard’s last visited an operating table.
Cross the Celtics off the list of possible teams to stop LeBron James’ seven-year run as Eastern Conference champion now that Irving has joined Hayward on the unavailable list.
The possibility certainly existed that the Celtics could get through the East with James’ Cleveland Cavaliers looking vulnerable and the top-seeded Toronto Raptors having plenty of postseason questions.
But without Irving, the Celtics are going to be in a struggle just to get out of the first round against either the Washington Wizards, Milwaukee Bucks or the Miami Heat.
A season that had so much promise when the Celtics reported for training camp in Newport, R.I., last September has been reduced to wait until next year because of injuries to their top two players.
Remarkably, the Celtics led the conference for a good chunk of the season after Hayward dislocated his ankle and fractured his tibia in the first game against the Cavaliers.
They have withstood a rash of injuries to put together a 53-25 record, trailing only the Houston Rockets, the Raptors and the Golden State Warriors, and they will be the No. 2 seed in the East when the playoffs get going next weekend.
But without Irving, who will wind up missing 22 games in his first season with the Celtics, they are headed for an unhappy ending in the postseason.
Irving last played on March 11 when he had to exit a game against the Indiana Pacers at halftime due to knee soreness.
On March 24, Irving had surgery on his left knee to remove a tension wire, which was put there in 2015 after he fractured the kneecap during the NBA Finals.
The Celtics said on Thursday that a bacterial infection had been found during that operation last month and now the screws are going to be taken out to make sure the infection is gone.
The team said Irving will “make a full recovering in 4-5 months’’ and that the fracture from three years ago “has completely healed.’’
Irving, like Hayward, should be ready to go when training camp for the 2018-19 season gets going in late September. For now, the Celtics, who signed guard Jonathan Gibson on Thursday, are going to be severely shorthanded when they play the No. 7 seed in the opening round.
“The hardest thing to do sometimes is accept the uncontrollable things life throws at you,’’ wrote Irving on Instagram. “You try consistently to learn, grow and prepare every day to equip your mind, body and spirit with tools to deal with some of those things.
`‘But I feel when those moments arise, they all give you a sense of unfulfillment, simply because it puts some of your professional journey and goals on a brief hold.
“This season was only a snapshot of what’s to come from me. Trust me.’’
The loss of Irving for the remainder of the year drastically changed the odds on the Celtics in the postseason. According to Westgate Las Vegas Superbook, the Celtics went from 20-1 to 80-1 to win the title and from 9-2 to 20-1 to emerge out of the East.
All of the high hopes that surrounded the Celtics this season are now on hold until their top two players are healthy.
Jim Fenton may be reached at jfenton@enterprisenews.com. Follow him on Twitter at @JFenton_ent.