Las Vegas Raiders coach Klint Kubiak noticed it while on a late-night walk to the steam room in the team facility. Defensive coordinator Rob Leonard saw it when he was on his way from his office down to the cafeteria to get dinner.

Rookie quarterback Fernando Mendoza alone on the indoor practice field going through the entire game plan. He would listen to the imaginary call in his head, communicate it to a huddle of teammates who weren’t really there and then go through a snap, dropback and full progressions.

Over and over. All by himself.

“When talent meets work ethic like that,” offensive coordinator Andrew Janocko said, “I think you have something special.”

The Raiders have been able to see it on the practice field — even as the No.…