I thought that would be your answer. What has he done to show that he can? His rating was 80.2 in very limited action this year. His career rating is 71.8 14 TDs to 17 Ints.
He's also started 17 games in his career. Including 400 yards against Minnesota as a rookie (a record) and solid games against the Bears and KC, both of which were lost by missed field goals by Rackers which Leinart drove the team into the winning position in....
Numbers for a rookie/young QB aren't much to put an argument on. He's looked good in limited action this year, and he's learning behind one of the game's all time greats.