Johnny Manziel is flashing shades of Matt Leinart -- and it's NOT a good thing -- so says Dean Cain -- who warns the QB that "going Hollywood" can backfire in a big way.
Cain -- who made it all the way to Bills training camp before an injury dashed his NFL hopes -- was in Malibu when he hailed Manziel as a great athlete ... "but there's a lot of athletes in the NFL."
That's when the Leinart comparison began -- "I do remember another quarterback back in the day, coming out and being a high draft choice and hanging out with the Hollywood starlets and stars and never really doing that well."
When we suggested that Matt -- who reportedly dated Paris Hilton and Kristin Cavallari -- was still living the life and scoring hot chicks, Cain shot back with a reality check:
"I'm not saying Matt Leinart can't get ladies ... but he cant get a contract in the NFL now."