We knew that he was going to go eventually, because of his quotes about playing in Argentina before he is too downhill. That way we knew he would not extend.
Therefore we should have done a couple of Lemos-deals the past 1 or 2 windows, so now, when we have to plan our CB-situation we already knew if he had proper replacements for the future. Now it is all being forced on us way too quickly, and even before the rumours about him leaving this or next year, I advocated for buying 2 CB's to make our situation more secure for the future in that position.
I think it is clear that we won't let him go this summer, and that he knows he can't just say that he wants to go 4 months later and then get through with it. I really thought we would have him until his contract expired, therefore had the time to bring in a successor, but now it is a little bit forced for my liking. Remember that a new CB won't necessarily fit just because he looks promising on paper. That is why we should buy 2 CB's even if he stays until the end of his contract, although it does not seem like it.
I bet I'm okay with signing someone we know we will get like Umtiti and a developing CB out on loan like Lemos or Meré or Davinson (I'm not sure why Lemos is our main target among them). But next summer, where Mascherano probably will go, then we need to bring in 1 more like Umtiti, and that could be one that requires more work, like Christensen from Chelsea (would be worth it - he is future WC CB + good cover at DM like Masch).