Ronald Gasser, 56, was just sentenced to 30 years behind bars ... less than 2 months after he was found guilty of manslaughter in a Louisiana courtroom. The maximum sentence he faced was 40 years.

Gasser was originally charged with 2nd-degree murder, but the jury elected for the lesser charge of manslaughter after more than 7 hours of deliberation back on Jan. 26.

McKnight -- who played for the Jets and Chiefs from 2010-2014 -- was gunned down by Gasser during a road-rage incident on Dec. 1, 2016, which ended when Gasser shot McKnight 3 times through his passenger side window.

Joe was only 28.

McKnight's mother, Jennifer, gave a victim impact statement before the sentencing, saying -- "That day, you didn't have to do that. That didn't have to happen. It hurts. It's going to hurt a long time."

Michelle Quick, the mother of Joe's 8-year-old son, also addressed Gasser before leaving the courtroom in tears.

"It will be my life's mission to make sure my son, Jaiden, doesn't grow up with hate for you. Not because you don't deserve it, but because he should not have hate in his heart."

FYI, Gasser's lawyer's filed a motion for a new trial before the sentencing -- claiming he was clearly acting in self-defense -- but the judge rejected their proposal.