Manziel attracted media attention after being pictured alongside New England Patriots star Rob Gronkowski and 10 bikini-clad models in Las Vegas, leading to a number of analysts questioning his commitment to his training schedule.
However, Pettine has defended the young Draft pick and stated that his off-field lifestyle will not be scrutinised so long as he is performing his on-field duties correctly.
Pettine told reporters: "Johnny has been great. He's in the playbook. Now if he was out doing something illegal, we'd have an issue. That just comes with the territory of who he is.
"I know he had his iPad with him. And I also know he wasn't using it as a coaster! But I guarantee you there were a lot of other NFL players in Vegas this weekend. And there were a bunch down in South Beach. I know some of our guys were down there, too.
"I can only judge him on what he's done when he's been here. I'm not going to judge him on how he lives outside the building as long as that's not affecting what he does here. And he came back here prepared and he's performed solidly."