Hard to say really.
Nobody really knows if he's really going to be sacked when we lose against Atletico. There might be rumours, but simply because of the transfer ban and the timing it's hard to think that our board would find a worthy new manager. Klopp is never going to leave Dortmund this season, Mourinho is not leaving Chelsea and so on.
Normally it'd be best to sack Lucho, get someone to manage the team until the end of the season and then get someone good but because of the ban that's not going to happen. 2016 is the first opportunity to really get someone and I wouldn't want to see a manager like Paco - who I think is brilliant btw - hired now but not able to build his team like he wants. He wouldn't make it to 2016 simply because he hasn't the reputation.
Therefore I think the earliest time Lucho would be sacked is if he loses against Atletico, loses in the Copa against Real/Atletico and loses in the CL against City. Signing someone like De Boer is pretty much the same as keeping Lucho. So it could even get worse if he's sacked (think about the possibility of someone like Roura or Eusebio), which is pretty sad.