What I'm reading into it is the O-line is simply in shambles right now and they need to get that sorted to even get a proper assessment of Rosen. That isn't going to happen by Week 1, the way things are looking.
Flores can't just come out and say that. It would be perceived as unfair (especially by Rosen's agent) to sit a player based on the failures of another unit...or the coaching staff itself, if you want to point to the recent O-line coach firing.
I want Rosen out there as much as anyone as I was a big fan of the trade when it happened, but the guy is going to get decapitated week 1 behind this line. WORST case scenario at the QB position this season is Rosen getting hurt before they can even tell how good he is.
Part of the reason he's "not ready."
Holding the ball a tad long.
Not stepping up in the pocket.
Not being decisive enough.
Etc.
All of that only makes the OL worse.
The plan is for him to improve in the above areas systematically.