Johnny Manziel: NFL Suspends QB 4 Games ... 'Substance Abuse Violation'

Might seem moot ... but Johnny Manziel will be suspended the first 4 games of the 2016 season for violating the NFL's substance abuse policy ... league sources tell TMZ Sports.

We're told the violation was NOT due to a performance enhancing drug.

Manziel has said he plans to make an NFL comeback -- but we're told if by some miracle he did get signed to a team, he will be forced to sit out the first 4 games of the season.

The good news, we're told the suspension does NOT roll over -- meaning if he signs a contract for the 2017 season, he wouldn't have to miss a game for this particular infraction.

We're told the suspension has nothing to do with Manziel's legal issues -- it was specifically handed down because of the substance abuse violation.