He's NOT a rookie. If he's ready, as far as grasp of the offensive scheme, play him. By all accounts (even those of his detractors), he is intelligent, so that shouldn't be an issue.
You, sometimes, sit a rookie to avoid having his confidence needlessly shaken. That ship has sailed in this case. He's already taken his lumps, and paid his dues, IMO, and it doesn't appear to have made him "gun shy", in the least.
If he's not ready, because of lack of understanding the O, that's a different story.
Miami is going to have at best two new starter at the O-Line (Probably 3), there is a new HC, O-coordanator, O-Line Coach, and if there is one good thing you can say about Fitzpatrick, he get hot early.
Believe me, I would love to see Rosen in there from day one, but this is not a "Let's show them how good Rosen is" moment, this is a lets be smart type moment, so instead, let's allow the line to get used to each other, allow them to get used to what the new O-coordinator and new HC is teaching them and wanting them to learn as far as offense, and allow ourself to try and ride Fitzpatrick as far as we can (if he loses games, then obviously, all bets are off) until the coaches feel Rosen is out playing Fitz in practice.
It's doubtful Miami makes the playoffs this year, but what if they actually do...
1. The defense plays well from the start.
2. Fitz is able to do enough with the defense playing well, to to win some games on that brutal 1st 4 games.
3. Rosen once he comes in, is everything we hoped, and more.
4. The coaching is actually really good, and press all the right buttons, make the right decisions.
They need to go into this season with a mentality that in the AFC east game one, they are tied with 3 other teams, and remember that less talented teams then the team Miami will be putting on the field did not just make it to the playoffs, but actually won it all (St Louis Rams as example).
Oh, and though i said 4 games. It could be less, it's up to the coaches when Rosen is ready. If he looks amazing in pre-season, then go for it.