In a surprising development, Celtics forward Gordon Hayward opted out of the $34 million player option in the final year of his contract Thursday on the eve of free agency despite his injury history and less-than-All-Star form of recent times.

Free agency begins Friday at 6 p.m., and the Knicks need a starting point guard most prominently, but they also could use a solid outside-shooting threat in the small-forward position even after the home-run drafting of scoring forward Obi Toppin.

Even though he is 30 and hasn’t regained his 2017 All-Star form, the Knicks are intrigued by the 6-foot-7 Hayward for the right price, according to sources.

The Knicks’ new assistant general manager, Walt Perrin, drafted Hayward, out of Butler, with the ninth pick in 2010 for the Jazz.

Because Hayward decided to opt out, he must know of teams willing to pay him at least between $18 million and $20 million a year on a two-year or three-year deal.

The Knicks opened $40 million of cap room Thursday with their expected waiving of most of their 2019 free-agent signings, including Taj Gibson and Bobby Portis. Both might be re-signed to much lower deals.

The Knicks are among three teams with a large swath of cap space in a free agency bereft of stars because Anthony Davis is expected to sign a two-year deal to return to the Lakers. That’s why Hayward stands out. The Ringer first reported the Knicks’ interest in Hayward.

Of course, the Knicks could use their foray into seeking a bigger fish to scare the Rockets into making a deal with them for point guard Russell Westbrook. The Knicks are one of the few teams that would have the cap space to take on Westbrook’s contract, but a Hayward signing would make it more difficult. The Knicks want Westbrook, but on discount.

The Knicks are in a win-now mode as evidenced by trading their 33rd pick in the draft for a 2023 selection because they didn’t want to waste a roster spot on a project.

According to league sources, the top free-agent point guard on the market, Fred VanVleet, is probably going to re-sign with the Raptors. The Knicks would rather not be used in that scenario and don’t want to give out a maximum contract.

In fact, they could get a serviceable starting point guard for value and they have three former Thibodeau floor generals — D.J. Augustin, Jeff Teague and Kris Dunn — on their radar. Each could be had for the range of $5 million to $7 million. The Knicks waived starting point guard Elfrid Payton on Thursday, when they declined his team option. He could come back if they strike out.

The Knicks need more outside shooting. Kentucky rookie Immanuel Quickley, selected with the 25th pick, is only a small start. Hayward’s career 3-point percentage is 36.6 percent.

Last season, Hayward averaged 17.5 points, 6.7 rebounds and 4.1 assists for the Celtics.

The Knicks have discussed Washington’s Devis Bertans as a perimeter threat, but their interest in Detroit forward Christian Wood appears to have lessened with the Toppin arrival.

In addition, Carmelo Anthony is on the radar, but the Knicks would have to hope he comes for a cheaper price, such as the $5 million room exception.