Matt Leinart -- FIRES BACK AT DEAN CAIN ... 'Stick to Being D List'

Matt Leinart isn't taking too kindly to being dissed by Dean Cain -- and now the former NFL quarterback is firing back at the "Superman" actor ... telling the guy to "stick to being D list."

Leinart is clearly pissed about the comments Cain made to TMZ Sports ... in which he said Johnny Manziel should be careful about running with the Hollywood crowd because he wouldn't want to end up out of the NFL like Matt Leinart.

Now, Leinart has gone to Twitter to respond to Cain -- and it's clear he's pissed -- saying, "Put 7 years in, pretty sure that's 7 more than 99% of the world."

Then came the personal shots at Dean -- a former football player who was cut by the Buffalo Bills in training camp after suffering a knee injury.

"Some people should stick to their day job because they couldn't cut it!"

He added, "Stick to being D list!"