So, how much is he going to fetch when he inevitably moves?
Right now, he is the starting striker of the 8th best side in the Prem, 4 goals in 16, in his mid 20s. If you had a striker with those circumstances not named Lukaku, I struggle to see he he should be worth anything more than 15-20m at best.
If he gets sold, it won‘t be in the next summer. Jose gets the sack at the end of the season for missing out on the CL, a new manager comes in and invests some money elsewhere. I doubt Lukaku would get sold without a big name CF coming in and I don‘t think the funds for that big name are there when you consider the gaping holes at CB, RW, DM, FB and probably even GK position. That means Lukaku gets kept and tried out, only to be dropped at some point during the season for some makeshift solution. That makes him a probable candidate for leaving the club in the summer transfer period of 2020.
I know I am working with assumptions, but history has shown us that this turn of events is indeed very likely. See how van Gaal didn‘t get rid of Rooney and Mourinho needed 1 full season to get rid of him, despite everyone knowing well in advance that he was useless.