Don't be.
We won't be able to find a like for like replacement for Lukaku. However, that doesn't mean we will be weakened. We will have money to buy 2 lesser players that can actually chip in with the same amount of goals as he would, and they may bring more to the team in other ways. We have relied too much on Lukaku in the past few seasons anyway with his good luck with injuries.
Lukaku is a top top goal scorer and deserves to be playing for the best clubs in Europe. He has given us 4 good seasons with plenty of goals but we're now in a position to move on and strengthen when he leaves.
There are only 2 or 3 clubs in the World that can hang on to their star players. Look at where Spurs are now compared to when they had Bale. Genuine title challenge last season, probably about to get in the top 4 again, and in the Semi Finals of the FA Cup.