I don't mind it.
I think you can take the quote a few ways.
It's either Koeman admitting that Lukaku will move on at some point, or him saying Rom is too big for Everton, depending on how you want to interpret it.
Personally, I see it as the first one.
It's nothing new to say that he will leave at some point if he continues this form. He's not going to be here for ever. Koeman is actually building Rom up ('he could play for Barca'), but at the same time saying he needs to improve.
While I don't like to hear it, it's the reality of our current situation. I'd rather our manager be acknowledging that than burying their head in the sand and trying to tell us that Rom will be here forever.