How SF handles him will tell you everything you need to know about Rosen.
they just signed him because they have 1 QB on the roster right now. They've been pretty clear they're going with Jimmy G, the injury to Mullens may change that in they might take a QB higher because apparently Mullens is having Tommy John surgery so he could miss all next year too.
Josh is not Shanahan's type of QB he likes to roll guys out, boot etc, Josh really doesn't move all that well, he COULD do that but it's not his thing I seriously doubt they'd do that.
Just the other day I watched a video of Peyton Manning explaining pre snap stuff where he was talking up Josh, as a rookie, cerebral guy, great arm, the Cards put so much on his plate for a rookie because they know how smart he is. It's amazing how fast he fell. We clearly saw something in our year with him and Miami clearly did too.
For us he just didn't get better, the OL sucked, we were signing Wr's from Wal Mart to play, but he just never seemed to have the game slow down. Same thing in Miami
If you watch just the highlights of his rookie season, it's a short tape but his arm and the ball he throws is so impressive, but he just never seemed to be able to deal with the pass rush, and the speed of the game. So many pick sixes.