If Fellaini is in form and still delivering he will still be first choice, but he's got alot of competition now. He will get less minutes probably next season, but can still be vital to the team.
Fellaini even as a sub would probably be higher in the pecking order than Hernandez for example. Van Gaal his plan A is to pass the opposition off the park, when at the end of last season we've seen van Gaal's plan B. Fellaini has unique strengths which are ideal for van Gaal's plan B: strong, good short passing, hold up play and especially headers to switch up the passing with long balls. Van Gaal used to send defenders to the box for this role, but now he's got a dedicated player for this. He will also be very handy for the games vs the Stokes of the league.