There is a big concurrence in Stade Français for flyhalf, as for all of the other positions in this club.
David Skrela is one of the top scorers of the last Top14 seasons and one of the most regular goal kicker. He is one the master key of the success of SF by his vision of the game and he is a strong defenser as well.
The raising star Lionel Beauxis who led France's -21 to the victory in world championship, has a real strong kick. He is able to score at 60 m. He doesn't care of the pressure.
The fact is Hernandez is a real great player, such a great one he can play everywhere in the backline. Fabien Galthié did not wait the RWC to think about making Hernandez at 10. JMH spent the last half of the season as the starting fly half of Stade Français, while Skrela was in 12 and Beauxis on the bench.
SF has a lot of very skillful players so Galthié has the possibility to turn them over without putting is side in a difficult.
So I think, in SF side, Hernandez's natural place is fullback, because of the lesser concurrence here, even if Jeanjean is recovering his top level. But it doesn't forbid him to play elsewhere