Even though Cooper Rush has killed it as the fill-in QB1 for the Cowboys ... Dallas legend Daryl "Moose" Johnston tells TMZ Sports Dak Prescott ain't in danger of being supplanted. "I'm a believer that you don't lose your job due to an injury -- especially if you were one of the starters, one of…

Three-time Super Bowl champ Daryl "Moose" Johnston wants Johnny Manziel far, far away from big D, because he's way too immature to handle the pressure of playing for America's team. TMZ Sports talked to the ex-Cowboy about rumors Dallas could be Johnny Football's next stop if the Browns cut bait…