Ex-NBA superstar Steve Nash STRONGLY believes Boogie can recover from his ACL injury and dominate in the NBA again ... and he's got the pump-up speech to help him bounce back!!!

"He shouldn't retire," Nash told us after leaving a pickup soccer game in L.A. ... "He's still a young guy."

Nash acknowledges DC's had a "horrible run" when it comes to injuries, but says the 29-year-old Lakers star has a "fighting spirit"

"He's a competitor. He's gone through a lot already. He can do it."

Cousins suffered his latest injury during a training session in Las Vegas earlier this month ... and footage obtained by TMZ Sports shows it was non-contact situation.

DC has dealt with injuries before ... he had a quad injury during the playoffs and dealt with an Achilles injury before that.

Nash says Cousins has proved he has what it takes to battle back -- and he hopes the guy will do it again.