The Boston Celtics’ ballooning payroll allegedly sparked a rift within the family that owns the team – leading the aging patriarch to demand his free-spending son sell the iconic franchise, The Post has learned.  

Irving Grousbeck, a 90-year-old Massachusetts-born entrepreneur who owns a controlling roughly 20% stake in the team, balked at funding big losses on the horizon from the massive contracts that helped the Celtics capture a record 18th NBA championship in June, multiple sources told The Post.

To win that title, his 63-year-old son Wyc Grousbeck put together the league’s priciest roster – one that is projected to cost around $500 million for the 2025-26 season after shelling out big-buck contracts this summer to lock up its stars.

Wyc Grousbeck — who owns a roughly 3% stake but is the front-facing owner — announced the surprise decision to put his beloved hometown team up for sale shortly after hoisting the NBA trophy.

“That’s what happens when dad puts in most of the money,” one of the sources said.

The team barely broke even last season during its championship run, sources said.

It is expected to lose roughly $80 million because of luxury tax fines for being over the salary cap for the upcoming season that tips off next month, a source close to the sale process said.

That figure likely will rise significantly in the 2025-26 season when harsher salary cap fines kick in.

The 11 players under contract — including stars Jayson Tatum, who got the NBA’s richest ever contract this summer, and Jrue Holiday, who also got a new deal, are due $225 million – putting the team on the hook for a $280 million luxury tax penalty, according to ESPN.

“Wyc says we’ll spend whatever it takes, but dad wasn’t into losing money,” another source said.

The Grousbecks have contended their selling the team for “estate planning purposes” when they made the stunning announcement.

Wyc Grousbeck reiterated that statement in speaking for the family, he said, when reached by The Post on Friday.

“The Grousbeck family is selling the team for estate and family planning considerations. To say the sale is in any way related  to losses is completely incorrect,” he insisted.

“There has not been a capital call from ownership, or any additional investment of any kind, in the 22 years since Boston Basketball Partners bought the team and we don’t anticipate there being one.”

Irving Grousbeck did not return calls and e-mails seeking comment.

The hallowed franchise of Red Auerbach, Bill Russell and Larry Bird was sold for $360 million in 2003 to the group led by Irving Grousbeck, a Harvard Business School graduate and Stanford University lecturer.

But part of the current financial difficulties is that they don’t own the Celtics’ home arena TD Garden, so they do not collect revenue from concerts and other events that would help offset losses, sources said.

The projected losses and lack of an arena to throw into the deal could make prospective buyers hesitant to shell out the $6 billion price tag the NBA reportedly wants the team to fetch as it looks to set a high bar for an expansion franchise in Las Vegas.

“This will be very interesting because the economics are not there,” one of the sources said.

“The Celts getting $6 billion is going to be difficult,” another source said. “It should be more like $5 billion.”

The Dallas Mavericks, who lost to Boston in the NBA Finals, sold for $3.5 billion last year.

That was short of the current-high $4 billion Mat Ishbia paid to buy the Phoenix Suns in 2022.

Earlier this week, ex-Milwaukee Bucks great Junior Bridgeman reportedly bought a 10% stake in his former team at a $4 billion valuation.

The proposed plan for the Celtics sale is a compromise between father and son in which they fork over a 51% stake in the next few months and then the rest in 2028.

Wyc Grousbeck would remain the controlling owner until the sale is complete.

Their bankers JPMorgan and BDT & MSD Partners have yet to distribute sales books, sources said.

The Post reported exclusively that Fenway Sports Group, owners of the Red Sox, are interested.

So is Stephen Pagliuca, the co-chairman of private equity giant Bain Capital who owns a little over 20% of the team, sources said.

NBA Commissioner Adam Silver does not like sales that happen in steps but indicated he would make an exception for this auction.

“We’ve said [to the Celtics] present to us what you’re considering and we’ll look at it on a unique basis. I think every one of these situations presents itself differently,” Silver said on Tuesday.