In my opinion Rom will only be fully appreciated once he's gone. His goals have bailed us out so many times over the last 4 seasons. Right now we have a border line world class striker. This is the hardest thing to get in football. Someone who guarantees you 20-25 goals a season.
In terms of a replacement the best we could hope for is a young player (I.e. the next Rom) who for some reason all the other big clubs won't take a punt on/whose scouts have somehow missed. Chances of this are small but I guess that's why we have people like Walsh at the club.
The safer (albeit less lucrative) way of replacing him would to go for someone already is ROM's class, but who is 29+ and after one last payday. The downside to this is that that there will be no sell on fee, the transfer fee will still be reasonably high, and the wages will be astronomical.
However, we will not be able to sign a player of Rom's ability, physical attributes, and age. It will not happen. Other clubs with bigger budgets and CL football will want these players and will get them before we do. I know we probably have more money now but Moshiri isn't writing blank cheques and will, it seems, only operate within FFP. If we have to play by the rules we won't get the top floor players.
I get what your saying that with the money we would get from the Rom sale we could re-build the team and improve the aggregate ability of the squad. But would that really work? Did it work for spurs when they sold Bale? What about the RS when they let Suarez go? Plus we would only get the big money if he signs the new contract. Without it he only has 2 years left. We'll only get 50mil for him I reckon. With a new deal it could be 70-80. We NEED him to sign it.
However, if I was Lukaku now there'd be no way I'd be signing a new deal with Everton. What's the point? A lower transfer fee will mean higher signing on fee/wages. Plus higher agent fees for Raola (whatever his name is). He's been here 4 years and we've done very little to push on. No one can accuse him of being disloyal at this point. If he had pushed for a move either last summer or the summer before he'd have definitely got one. Maybe not to a Bayern or Real (where he ideally wants to go) but certainly to somewhere like PSG or Athletico.
The very best we can hope for is he signs and gives it one more year. If we are not in the top 4 in May 2018 he's gone. If he does sign we need to do everything to show him our ambition. We've shown a bit of ambition over the past 12 months but when you remove all the Jim whites quotes and transfer rumors and general meetings it all boils down to a bit of cladding on the stadium, Koeman's massive wages, Martinez's compensation, and Morgan Schneiderlin. This is not enough to push on and certainly not enough to keep a player of Lukaku's class.
