£40m is probably the best price we can get for a 29 year old with 2 years left on his contract. I'd rather sell him abroad for 30 mill, but I haven't heard of anybody being seriously interested other than United, so this may be our best chance of significantly cashing in from a player who probably wants to leave. And on the positive side we can use that money to finance the signings of other potential targets that Conte really wants.
Ideally of course I would not sell him at all and keep him to compete with Bakayoko. Selling him also means getting another midfielder unless Conte puts a lot of trust in Chalobah/Baker/RLC, which I doubt. Having only Bakayoko, Kante, Fabregas, Chalobah is far too thin and would once again put us in a place where one injury/suspension would set us back to stone age. So hopefully the money will be well reinvested in midfield.