It's the question everyone wants to know after he essentially disappeared from relevance in the NFC title game and the Super Bowl ... and even NFL insider James Brown says something's not right.

The L.A. Rams running back was basically unstoppable during the regular season -- and made a legit case to be the league MVP.

But, he faded quickly during the NFC title game and barely played in the Super Bowl ... despite the fact both he and coach Sean McVay insisted he was NOT restricted by any sort of injury.

So, what's REALLY going on with Gurley?

"I don't think we know the full story as far as Todd Gurley is concerned," the "Inside the NFL" host told us ... "There's a lot more to that."

Brown continued ... "Sean McVay and the coaching staff understand everything that he can do and to the degree that he was an MVP candidate throughout most of the season, and he didn't get played that much during this stretch here. There's more to the story."

After the Super Bowl, some people expected Gurley to drop the bombshell that he was suffering from an injury that he was trying to play through ... but that wasn't the case.

Gurley told the media, "I felt good ... whenever my name is called to get in, I'm ready."

McVay also reiterated he was healthy enough to go.