Brian Hoyer has guided the Browns to a 4-3 record so far this season while throwing for eight touchdowns and two interceptions.
"That's a position that can be frustrating," Pettine told the media.
"He's preparing like a starter. It can happen in a span of one play. He has to be ready to go, has to be prepared. There's no reason to believe that he's not."
Manziel has publicly supported Hoyer and he insisted last week that he needed to focus on "playing [his] role as back-up and that's that".