It's a no-brainer for Rosenior. For example, Fabregas/Iraola/Glassner are on-demand names, if I was them I would seriously consider wheter Chelsea is right step or should wait for other oppurtunity. But what is Rosenior risking? I didn't see his name circulating around top teams, not even Premier League.
Pay rise, chance to shine, and only can benefit from it. If he fails, he can still find Champo/French league jobs, if he (even moderately)succeeds, his value can rise, his carreer can lift to a new level.
Better wait for the official announcement. I still remember for the Omorodion incident..