Kevin Durant and Russell Westbrook can only wonder how their future might look had James Harden remained in an Oklahoma City uniform. (Soobum Im/Usa Today Sports)

As the San Antonio Spurs continue to play brilliantly, the Oklahoma City Thunder potentially faces a much bigger threat to its future than merely being embarrassed in the Western Conference finals. With the Thunder trailing, 2-0, in the best-of-seven series, league MVP Kevin Durant could be on the verge of another frustrating playoff exit.

You don’t have to hold a high-ranking position in the Thunder front office to realize that winning a championship could significantly aid the organization in its efforts to retain the seven-year veteran, who can become an unrestricted free agent after the 2015-16 season. Durant strives to be the best. If the Thunder fails to reach the top before his contract expires, he may try to get there with another team. For the Thunder, the thought of Durant leaving, rightly, is even more unsettling than being disassembled by Spurs point guard Tony Parker.

Not too long ago, Oklahoma City seemed destined for a long title run. Back in the 2011-12 season, the young Thunder lost to the Miami Heat in the NBA Finals. After the five-game series, Miami megastar LeBron James told Durant, his friend and occasional workout partner, “See you here next year.”

Both teams were constructed, albeit differently, to win multiple championships. Last season, Miami got its second consecutive, rallying to defeat San Antonio in an epic series that went the distance. In that postseason, the Thunder was derailed by Russell Westbrook’s knee injury. Now, Oklahoma City is in trouble without injured shot-blocking big man Serge Ibaka, who has been ruled out for the remainder of the playoffs.

The Thunder is a much different team without Ibaka, who finished second in the league in blocks, watching its back near the rim. Although Parker could teach a course on how to ruin an opponent’s best-laid defensive plans, he has sped to the basket more than usual against the Thunder. Unless the Thunder quickly shores up its interior defense, which isn’t likely to happen without Ibaka, the team will be on vacation soon no matter how well Durant may perform.

Would a rematch of the 2013 NBA Finals be better than a possible LeBron vs. Durant matchup on the NBA’s biggest stage? The Post Sports Live crew offers best-case scenarios for the NBA Finals. (Post Sports Live/The Washington Post)

Even with Ibaka, the Thunder isn’t as formidable as it once was. There’s no other way to put this: The James Harden trade has been a disaster.

Before the 2012-13 season, the Thunder sent Harden, the NBA’s sixth man of the year the preceding season, to the Houston Rockets in exchange for Kevin Martin, Jeremy Lamb, a 2013 first-round pick, a 2014 first-round pick and a 2013 second-round pick. The Thunder had already signed Durant, Westbrook and Ibaka to massive multiyear contracts. Thunder ownership, which had luxury tax concerns, declined to reward Harden with the maximum contract he wanted. The sides failed to reach an agreement, resulting in Harden’s departure.

As expected, the fast-rising Harden immediately became a superstar in Houston. One of the NBA’s top scorers, Harden got a maximum deal and has proved he deserved it.

After only one season, Martin left Oklahoma City in free agency. This season, Lamb fell out of the rotation in early April. In the playoffs, Lamb has played little.

Rookie 7-footer Steven Adams, chosen with the first first-rounder acquired in the Harden deal, is contributing. The 20-year-old New Zealander looks like a keeper. But you don’t trade a game-changer like Harden, who could fall out of bed and score 30, for an energetic big guy who sets screens well. That brings me to Thunder General Manager Sam Presti.

The man who drafted Durant also will play one of the biggest roles in trying to persuadeF him to stick with the Thunder. Among top team builders in professional sports, Presti is a rock star. He has a keen eye for talent, as evidenced by the Thunder’s many home runs in the draft. His track record speaks for itself. Of course, no one’s perfect.

It appears Presti made a key strategic error in locking up Ibaka before Harden. In Durant, Westbrook and Harden, the Thunder had three players capable of being the focal point on offense. A season ago, the Thunder may have overcome the loss of Westbrook and made it back to the NBA Finals if Durant had Harden to help him. Harden is that good — especially when teamed with Durant. Fact is, shot blockers are easier to find than highly skilled wing players capable of taking over games.

I really enjoy speaking with Presti. I’ve talked with him about the Ibaka-Harden decision and understand his thinking. Ibaka is an animal on defense and an emerging force on offense. And the Thunder did offer Harden $55.5 million. That established, in essentially picking Ibaka over Harden, Presti may made a move that ultimately could lead to Durant’s exit.

The Thunder should be one of the NBA’s top teams the next two seasons. Despite Westbrook missing 36 games while dealing with knee issues, Oklahoma City still went 59-23 in the regular season. It finished only three games behind the Spurs for the top seed in the West. It’s just that Durant won’t be satisfied until he wins the game’s biggest prize. James has been there.

During his first seven seasons, the Cleveland Cavaliers failed to build a championship-caliber roster around James. Figuring he couldn’t win a title in Cleveland, James went somewhere he thought he could. With the Heat, James has a chance to make it three in a row. Durant wants to play where he can achieve similar success. So far, Oklahoma City hasn’t been that place.

Since Durant signed an extension in the summer of 2010, the Thunder has been up against the clock to build the type of team he would want to sign with again. While they continue to work on it, time is running out.

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