The idea that Fellaini is only useful through lumping the ball forward in a desperate final attempt to score does him a disservice. While his position is definitely as an advanced player and he can cause bother in the air in the final third, he has more to his game which allow us to play more attractive and effective football than that.
Fellaini is very good at receiving the ball with his back to goal, proficiently making it stick even when under pressure and taking the ball in at awkward heights. He is more intelligent in advanced midfield or forward positions than given credit for which, married with his industry off the ball, often leads to him unsettling opposition defensive lines - dragging midfielders too deep or drawing defenders out too high. There is something unfussy about his hold up play too, almost never holding onto the ball too long - he kills it and moves it on. Fellaini knows exactly what he’s capable of and plays to his strengths in the final third.
Using him in this way, he provides an effective way of progressing the play. It is always good to have that forward passing option - someone who can stretch the play and take the ball in under pressure, giving the team a target to pass and move off of and a way to bypass aggressive pressing.
We should never just play long balls into him from deep, instead look to use his abilities in build-up play and work the ball into good positions around the opposition box. If we do that then his strength, height and general awkwardness in the air becomes more pronounced. Rather than looking to flick on the ball from deep, relatively straight long passes with his back to goal, often around the edge of the box, there will be more opportunities to come onto crosses in the box and head at goal or knock down into more dangerous areas.
The period of time where we have most ably exploited all of Fellaini’s abilities was during the small flurry of good performances under Van Gaal, where he played in a high inside left position in midfield and constantly created a triangle with Young and Blind down that side. They constantly used him as a link man to work the ball forward and move into attacking positions and, coupled with Herrera, Mata and Valencia linking in their own way down the right, for a small period of time(perhaps the only time under the Dutchman) we had a good balance to our side and played effective football.
With Mkhitaryan struggling and Mata being similarly poor out wide and having seemingly no opportunity to play centrally, I would have no issues with Fellaini taking that no.10 spot as a starter as things stand. For me, that would be the right way to go at the moment.
The question, in my mind, is around Zlatan’s return. The qualities I’ve described Fellaini as having are pretty much all possessed by the Swede. Not quite as much of a nuisance at the back post and not always as keen to release the ball quickly or bust a gut to get in the box. A supreme technician and a genius though - far sharper, much more precise and imaginative with the ball, and with a natural goalscorer’s instinct. He is, quite simply and still at the age of 35, a much better player - a top player who provides a little bit of the stardust we’ve been missing. While I would want him in the side in that position, I’m not convinced he would defend with the required diligence. Lots of question marks. Will he be fit? Will he be the same player we had last year? Could he play slightly deeper, consistently?
How long will Ibra stay for? If he won’t be here next season then I think it would be prudent not to lose a player of Fellaini’s qualities. While I, like basically everyone, would prefer to see a more technical and creative player as a first team regular there - better to have someone with a creative spark, especially since Lukaku does not - as an alternative I think he is great. You will never have a squad with a like for like replacement for every starter, it is simply not possible. A backup should offer something different and Fellaini is a fairly unique player for that position, who will ask new questions of any opposition when called upon. Money is not as valuable to United as having that option off the bench or in certain games.