You're just presuming that.
I do worry about Rosen behind this offensive line, I can't lie, but I've learned you can't be afraid to take a chance on a QB just because of your line. Though we seem to be catastrophically bad at it, its easier to find line prospects than QB prospects. I've seen this team field unfathomably bad lines before that I thought nobody could be successful behind, then I watched guys like Pennington and Frerotte come in and actually move in the pocket behind most of those same linemen and suddenly our line still wasn't good, but it wasn't something you had to think about. Bad QB's and bad staffs make lines look worse than they are. Remember how much better the line looked in a lot of games Osweiler started (note, not calling him a good QB)?