He wouldn't have signed the contract if he didn't want to stay long term and it would be extraordinarily stupid if Roma gave up one of their best players just because he doesn't really fit with a coach that'll only be there for one season.
I doubt Roma would reject any big offer for De Rossi, but they would need De Rossi to request the transfer for them to stand a chance of not being lynched by the fans.
Aaand here comes very bad news for Milan! We are only 4 points away from breaking Milan's outstanding "most points in Italy from 2010/11" stat!
Since 2010/11, Milan have 82 + 80 + 14 = 176 points. Juve have 58 + 84 + 31 = 173 points.
But there's still hope for them, they can start saying 'most points in Italy from 2009/10"
He wouldn't have signed the contract if he didn't want to stay long term and it would be extraordinarily stupid if Roma gave up one of their best players just because he doesn't really fit with a coach that'll only be there for one season.