Fitzpatrick can play behind a terrible line. Rosen cannot. Fitz knows multiple systems and has been around long enough that when things break down he can improvise. Rosen cannot at this point. When that line isn't working, it basically becomes back yard football.
Rosen has played 20 games in the NFL. All of them behind atrocious lines. He has never had a game better than a 88.9 QB rating and that was last year vs the Chargers. He has only 3 games in which he reached an 80 QB rating. Out of those 20 games, he has a 60% completion rate in just 5 games. His career comp % is 54.8% with a career rating of 52.
By way of comparison, as a rookie Tannehill had QB ratings of 112, 123, 93.7, 90.9, 92.3, 91 & 97.1. (7) games above 90.
Take the numbers however way you want but what it tells me is Rosen was over-drafted and not ready to play in the NFL. That does not mean he cannot be a good QB someday but he definitely needs some major coaching to get there. For all we know Aaron Rodgers would have started the same way but he got to sit for 3 years and learn. You just never know.