A number of 2018 first-round prospects -- including potential lottery pick Mitchell Robinson, who withdrew from Western Kentucky before the season started -- have inquired about the possibility of playing in the G League this season but were told they are ineligible, a league source told ESPN.

That ineligibility stems from a rule that prevents players who were enrolled in college during an academic calendar year from being offered a contract in the same season, unless they have been ruled permanently ineligible by the NCAA with no opportunity of being reinstated (as was the case with P.J. Hairston in 2013).

The rule reads in full:

"A player shall be eligible to be signed a (G League) Player Contract only if he has satisfied all applicable requirements of Section (a) below, and one of the requirements of Section (b) below:

"a. The players (i) is or will be at least eighteen (18) years of age during the calendar year in which the Player Draft is held, and (ii) with respect to a player who is not an International player has graduated from high school (or if the player did not graduate from high school, the class with which the player would have graduated had he graduated from high school has graduated); and

"b. Either (i) the player has NOT attended a college or university in the United States or Canada during the academic year that takes place during all or any part of the season; (ii) the player has attended a college or university in the United States or Canada during the academic year that takes place during all or any part of the season but is no longer eligible in the current academic year (including by enrolling) to play basketball for the college or university during the season at the time of signing the Player Contract; or (iii) the player has no remaining intercollegiate basketball eligibility. "

Players who don't fall into this category include those with ties to the federal investigation into corruption in college basketball. That pool that includes De'Anthony Melton (USC), Brian Bowen (Louisville), Austin Wiley (Auburn) and Danjel Purifoy (Auburn).

It also does not include players who have elected to withdraw from college voluntarily, such as LiAngelo Ball.

Since the NCAA has not ruled on the eligibility of players related to the FBI probe, these players have the option of signing overseas, spending the entire year training in America and entering the pre-draft process with no real résumé, or waiting for an NCAA ruling.

Playing in the G League is not a possibility.

This rule has been called into question by numerous NBA front offices -- as well as player agents. They say they don't understand why the G League is preventing legitimate draft prospects from being seen by scouts and executives in the U.S. Some went so far as to question whether the NBA is colluding with the NCAA to protect the interests of college basketball.

During the same weekend of the annual G League Showcase in Toronto, more than 40 NBA scouts and executives came to attend a prep school game on a tiny Lifetime Fitness basketball court to evaluate second-round prospect Matur Maker, the younger brother of Milwaukee Bucks center Thon Maker. Many NBA front offices consider it a missed opportunity that the likes of Robinson, Melton, Bowen and Wiley weren't able to play at the Showcase, allowing a rare look at a group of potential first-round picks competing against real professional players.

The G League had no official comment to make on the record. But a league source provided this reasoning, which was reinforced by multiple sources.

"We're not looking to compete with college basketball for their players," the league source said. "The NBA, specifically NBA lawyers, are concerned about the optics of NCAA players being disgruntled with minutes or coaching decisions and leaving college with the hopes of joining the G League. This is a blanket rule unfortunately that applies to all players. Like all of our rules, we are open to revisiting them if needed, but at the moment any player that was enrolled in a college this season is ineligible to play in our League."

One NBA general manager, who requested anonymity, disputed this notion.

"Is it really the NBA's place to tell players what they should or shouldn't do with their lives before they enter the draft?" the GM asked. "Normal kids drop out of college every day for a variety of reasons, and we don't put any restrictions on what they can do afterwards, as long as they are qualified. Brian Bowen and De'Anthony Melton didn't do anything wrong and shouldn't be punished because of the actions of their family or friends.

"These guys are in limbo right now and should be allowed to play somewhere in the U.S. ... I'm not even sure this would hold up in court, honestly, if someone challenged it. The sad part is that the NBA Players Association has no jurisdiction over these kids before they enter the NBA and has no say in the G League bylaws."

NBPA official Elle Hagedorn confirmed that there is nothing the union can do.

"This is an issue that the NBA will have to speak to directly," Hagedorn said. "This is their rule and not one we bargain over as the union. We do not represent the G League players and therefore have no input as to their rules and regulations (except for the NBA players assigned there, who are still covered by the protections of our CBA)."