In a perfect world he wouldn't have to. But he's got a release clause so someone COULD take him against our wishes; Furthermore we need the cash from his sale to improve the size and depth of the squad and to create competition for existing players. Without selling Mofro, we have about £9m to play with, tops.
If he goes and generates £23m, that could give us a pot of £30m-ish to rejuvenate our squad. At the risk of getting all Champ Manager:
Dragovic: £5m
Fer: £8m
De Guzman: £6m
Belhanda: £10m (with Gueye going to Montpellier)
That's four players that could all conceivably take spaces in the starting eleven, generating competition for places with the likes of Distin, Gibson and Osman. Also provides cover at RM, our weakest position.
For $hits and giggles we could also sell Heitinga for £3m, loan Duffy out, sign Douglas on a free from FC Twente and have a couple mill in change.
But none of it can happen unless we raise funds by selling Fellaini.
I think your wrong that Fellaini HAS to go due to us needing the money for squad depth. I would imagine we have around 15M to play with, Could even be more when you look at what the likes of Norwich and co are throwing around already! Also remember a lot of deals are done on tick these days....
Goalkeepers - Only got Howard so yes one is needed. That could be a free, loan or indeed cash spent. But you have options which way to go this summer.
COST - £0 - Upwards
Defense - IMO we need to move heitinga on and replace with the money his transfer brings in (roughly 3M) you may have to add a million or 2 out of the kitty but there are also potential free deals to be had.
COST £0 - 3M
Midfield - As it stands centeraly inc the No.10 role we have Gibson, Fellaini, Osman, Barkley. I personaly think we only need to bring one money signing into that. A (Fer or Honda) at around 5M would complement them well. Of course 1 or 2 more needed around the squad needed to cover for injuries but free's/loans even (lundstrum) could easily bulk that up.
COST £5 - 8M
Wingers - Deffinately short here, only really Miralass and pienaar. Could bring in a few Million from Naismith so with free's (Donovan) we could potentially add another couple of squad wingers for a another couple of mill from the kitty.
COST £0 - 3M
Strikers - 100% need to buy one. Dependent on what's done with winger additions we could sell jelavic and add 5M or so from the kitty and buy a 10-12M striker, still having super sub vic and wingers in mirallas and Donovan who can play up front also.
COST £5M - 10M
General point being we could add what is required without having to sell the big man and be a stronger team/squad. If he wants to go then not much we can do but a few savvy transfer dealings in and out could make a real difference.