Mirallas cant lead a line and neither can Kone. There is no able deputy at the club for Lukaku if he gets injured.
Yarmolenko would be a welcome addition, but we're not likely to get him. They're asking big sums for a player who's near the end of his contract and he;ll be off to a club in the CL anyway.
He's not near the end of his contract, he signed an extension.
And again, regardless of who we signed, Shane Long or whoever - nobody could come in at this point and offer half of what Lukaku is doing, so we'd be relying on him not getting injured either way.
So the choice is bringing in someone who will deputise for Lukaku, or someone who will make Lukaku even more effective. I'd go with the latter.