Gah, seems like this has to be repeated over and over again whenever Fellaini is discussed, but it isn't necessarily about being 'better' or 'worse' as standalone terms in a diametrically opposed sense; and the discussion inevitably hits a cul-de-sac when one uses reductive black and white terms like that to quantify things, without looking at the team as a whole and questioning why the players who're supposed to be among our best aren't performing at an optimal level. It's not about Fellaini being our apparent best player, or in form - that's not how you 'build' a team as a new manager, and that's so not how you build an identity for subsequent sides. Someone like Fergie could pull that off because we already had an identity, and he could easily mix and match things. But not someone who's just walked in that door.
You build a team to highlight the best features of your best players (in terms of talent and technique), not stand pat because one of the worst players in your team (again - in terms of talent and technique) is playing at a decent level. If it was only about being good in the moment and in current form in a team that is performing rather poorly, then you can't possibly build a stable lineup, because form in transient and you're not actively striving towards a pattern that brings out the best in your best players. To build a holistically sound midfield unit, you have to have complementary features - it's a basic concept. It might be crap at first, and you might have to rip things apart before rebuilding them instead of seeking the easy out; but if it's fundamentally and structurally spot on (or close to it), you can 'play people into form' with more game time - it's a matter of conviction. Then, you have the proper structure and players in form.
eg. 1: There's a guy named Andrea Pirlo who wasn't much cop at Internazionale. We've bought him, but we have no fecking idea what to do with him. He isn't a top quality CM in the all-round traditional sense, or an AM that can take over games with pace and goalscoring ability. But we think he can be a good deep playmaker - someone who can control the game for us. He has excellent deep passing ability and positional/tactical awareness, but he's soft and slow and weak (which is why his previous team team could't find ideal use for him). We think we have to persist with him because of how good we think he can be, but how can we accentuate his potentially world class ability on the ball in terms of dictating the game? We put a mobile and complete box-to-box alongside him in Seedorf, and we put a selfless harrier on the other side in Gattuso. Now, we're minimizing his weaknesses - the midfield is no longer soft and slow and weak as a collective; and he can dictate things while sitting back. And since the other two don't want to control the game, Pirlo's left to do what he's actually good at.
eg. 2: There's a guy named Paul Pogba who was pretty groovy in Juventus. Not so much for France though, because he was being asked to play in a central, more tactically disciplined role. We've gone and paid the world record fee for him, but we insist on using him like France (because that worked out so well, didn't it?). We know that he isn't a top quality dual CM in the traditional sense, or someone who can dictate the game. But we do know that he's world class as a 'spare' midfielder who can transition into the final third. He has excellent physique and pace and technical skill on the move towards goal; but he's tactically underdeveloped as a CM, a bit naive in terms of tracking back, sometimes he shows a lack of effort and dawdles on the ball - which the previous team compensated for by putting him next to a tactically astute albeit not very mobile player who will reduce Pogba's tendency to sit on the ball by controlling how much of the ball he sees (Pirlo/Marchisio) apart from a mobile midfielder who tracks well and is good at pressing teams (Vidal/Khedira). But nah, what do we do? We put him next to a past it #10/SS/CM/Libero/quarterback; and Fellaini - who's tactically and technically stunted. Then we wonder why our world record signing is playing poorly...
It's so not about Fellaini being in form/our of form or good/bad. It's about building things in a way that accentuates the strengths of your best players in terms of talent and actual, proven pedigree (Zlatan/Pogba/Mkhitaryan among others). You put Fellaini in midfield, and because he's not positionally astute and can't pass at anywhere above a rudimentary level, Pogba's left to do the passing and watching his movement - which results in crap because he's not very good at that:
We know that he isn't a top quality dual CM in the traditional sense, or someone who can dictate the game. But we do know that he's world class as a midfield third who can transition into the final third. He has excellent physique and pace and technical skill on the move; but he's tactically underdeveloped as a CM, a bit naive in terms of tracking back, sometimes he shows a lack of effort and dawdles on the ball
So you're telling you shiny new toy to suddenly develop new skillsets at a great level to compensate for Fellaini (the best player in the team). All the while, you're overlooking someone like Carrick who's:
the previous team compensated for by putting him next to a tactically astute albeit not very mobile player who will reduce Pogba's tendency to sit on the ball by controlling how much of the ball he sees (Pirlo/Marchisio)
And the other cog is right in front of your eyes too (Ander):
apart from a mobile midfielder who tracks well and is good at pressing teams
The solution is pretty simple, but you don't wanna do it because Rooney plays almost every game, and Fellaini 'is our best player' and others aren't stepping up - instead of reasoning why they aren't stepping up - they aren't stepping up because football isn't all about blood and guts, but about systemic fits and specific players traits - that you try to harmonize in an defined setup. No wonder we look positionally and structurally inept, and lack a real identity. Put your best players in positions of strength and surround them with the obvious supporting cast instead of half-arsing things and suddenly expecting them to step up and change their games to compensate for players that shouldn't even be a point of emphasis. The fact that Fellaini is playing well while your record signing isn't playing up to par for obvious tactical/individual player characteristic reasons should send alarm bells ringing. Or, you could be satisfied with Fellaini as the DM; and hope that Pogba suddenly becomes much better at controlling the game, tactically responsible and good at tracking back to cover tactical flaws. Dunno, maybe I'm a raving loon who's oblivious to the virtues of Fellaini as a starter in this team.