The NBA is investigating whether the Philadelphia 76ers and guard James Harden are in violation of the league's new player participation policy for their opening night national television game against the Milwaukee Bucks on Thursday, a league spokesperson told ESPN.

"We are looking into the facts around James Harden's availability tonight to determine whether an approved reason exists for his lack of participation," league spokesperson Michael Bass told ESPN.

Harden's situation is an interesting early test of the league's new legislation meant to increase star players' availability for national television games. The Sixers decided to keep Harden in Philadelphia after he returned to the team following 10 days away with what the team called "a personal matter." The Sixers believed Harden could get himself into game shape more efficiently by remaining at the team's practice facility to work with the Sixers' development coaches and medical staff, sources said.

Harden had wanted to travel to Milwaukee and Toronto on the team's opening road trip, but the Sixers didn't believe he was physically prepared to play after spending so much time away from the team, sources said. Beyond his 10-day absence from the team, Harden's lone preseason scrimmage was Oct. 7 in Boston, which further clouded his readiness to make his season debut, sources said.

The NBA's player participation policy allows teams and players an out for injury, personal reasons and "rare and unusual circumstances." A team could be fined $100,000 for a first offense, with increasing penalties thereafter.

It is believed that Harden's decision to be away from the team stemmed, at least partly, from his displeasure with the fact that the organization hasn't yet honored his request for a trade to the LA Clippers. Harden exercised his $35.6 million player option for this season and immediately requested that the team trade him.

After months of negotiations, the Clippers stepped away from Harden trade talks in recent days, sources told ESPN. The Clippers could revisit them later in the season, but the organization wants to take time to evaluate the team around healthy stars Kawhi Leonard and Paul George over the next several weeks, sources said.

ESPN's Bobby Marks contributed to this report.