Roger Goodell's explanation of why Colin Kaepernick's NOT in the NFL is trash ... 'cause he's better than EVERY NFL backup QB and some starters, so says Reggie Bush, Kap's former 49ers teammate.

Translation: Colin's wayyyyy good enough to help an organization win. So, not being signed ... it ain't about talent, or W's and L's.

TMZ Sports saw Bush -- who knows good QB play (he spent 5 seasons with Drew Brees) and winning (he's got a Super Bowl ring) at Shaq's Fun House bash in Atlanta -- and asked him if Goodell was right about Kaep.

Remember, earlier this week Goodell was asked about Kaepernick not being in the league ... and the commish pointed his finger at teams, saying they'd sign Colin if they thought he could help them win ball games.

Nonsense according to Bush -- who was with Colin in San Francisco in 2015.

"It's not true. It's just completely not true," ... Reggie told us.

"There are 32 backup quarterbacks. Kaepernick is better than all 32 backup quarterbacks, and he's better than some starting quarterbacks. I can name a few, but I won't do it for their sake."