Inside the Knicks’ attempt to keep Isaiah Hartenstein and how they may replace him after Thunder deal

The money talked and the Knicks didn’t have enough.

Isaiah Hartenstein agreed to a three-year, $87 million deal with the Oklahoma City Thunder on Monday morning, multiple sources said, leaving the Knicks with a hole to fill at center as they navigate the remaining lower tiers of free agency.

The Knicks gave it their best effort to re-sign the starting center, offering their maximum of four years, $72.5 million as soon as the NBA Finals finished.

That offer remained on the table until Hartenstein agreed to the balloon offer from the Thunder, which is front-loaded and not guaranteed in the third year, a source told The Post.

Hartenstein’s salary next season will hover around $30 million, whereas the Knicks could only start at a shade more than $16 million in Year 1 of a new deal.

Hartenstein, per multiple sources, wanted to return to the Knicks, but multiple factors made OKC’s offer impossible to turn down.

First, the Thunder, while maybe not guaranteed to have a starting spot for Hartenstein because Chet Holmgren plays center, are a legit championship contender after winning 57 games last season. OKC’s biggest issue was rebounding, and Hartenstein, a bruiser in the paint, will help alleviate that problem.

Second, the money is not just about the salary, but also the cost of living/taxes for a fresh first-time father who spent much of his NBA career on relatively low-paying and non-guaranteed contracts.

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According to Spotrac, Hartenstein’s career earnings since 2018 are $22.65 million. He’ll easily surpass that in one year of his new deal.

The reason the Knicks were limited in their offer is because they’re in the luxury tax and have no cap space. Their only avenue toward re-signing Hartenstein was using his Early Bird Rights, and the market dictated that wasn’t nearly enough.

After Nic Claxton re-signed with the Nets, Hartenstein became the No. 1 center on the market. As The Post reported, OKC, historically a tough place to recruit free agents, was viewed as the “top threat” to pry away Hartenstein.

Then, the Thunder executives made the 26-year-old a priority by traveling to Eugene, Ore. – where Hartenstein was born and still has family – to meet with him in the opening hours of free agency Sunday.

So, where does this leave the Knicks? They still have an obvious replacement at starting center in Mitchell Robinson, whose biggest drawback is his history of health issues. Robinson missed a combined 74 games the last two seasons and is coming off surgery to his ankle.

Even when available, he’s only averaging about 25 minutes.

They recently picked up the contract option for Jericho Sims, as The Post first reported. He’s raw but with upside in his athleticism and defense. Tom Thibodeau started Sims in 27 games over the last two seasons.

They still own the Bird Rights to Precious Achiuwa, who is probably more of a power forward but showed aptitude at center last season when thrust into the mix because of injuries.

Center depth is the one glaring hole in the Knicks roster construction. They used one of their draft picks last week on a center, taking 7-footer German prospect Ariel Hukporti at No. 58.

The free-agent market for centers is pretty slim and the Knicks, if they stay below the second apron, will only have access to the taxpayer midlevel exception (roughly $5 million starting salary) and just minimum contracts after that.

Guys such as Mo Bamba from Philly and Goga Bitadze from Orlando were still on the board as of Monday morning and Charlotte’s Nick Richards could be a trade target.

Others – Jonas Valanciunas, Andre Drummond, Drew Eubanks, Alex Len, Mason Plumlee, Luka Garza, Richaun Holmes – signed elsewhere in free agency.

Losing Hartenstein was inevitable, in the end. He became a lesson in development from Thibodeau, who has taken undervalued centers (Nerlens Noel, Joakim Noah, Taj Gibson, Robinson, Hartenstein) and got them all paid big bucks.

But Hartenstein was more than just the latest center triumph from Thibodeau. He was a strong fit with the current Knicks — a physical paint protector with a growing role in the offense — with the analytics backing up his impact on winning.

Hartenstein will be very difficult to replace.