I don't think the club would do that to any player. It sends bad signals and would make incomings harder. If Mendy has to leave, they will find a way, be it a sale for less or a loan. Imo, he would be replaced by another left back and unless it is from the academy, still a space. What order that happens though, fair point.
I guess I am responding because I make a point of squad spaces needing made for Graelish to arrive, so I feel I may be one of what you refer to. It is not an excuse for not wanting Graelish, it is just a point that for him to arrive, something needs to happen and one needs to leave. And often I say it in response to people claiming he can just show up without affecting the squad because the foreign quota is misunderstood to have a few spare slots.
The club will obviously reshuffle the squad holistically to het what they want, over the whole transfer window.