These are going to be very important weeks for Barcelona and especially for Koeman. I don't really like him as a manager and I think if he loses the Clasico now and then maybe has another fail in the next game in the Champions League, there are even tougher times ahead than is already the case. How would the club stick to Koeman without their fans freaking out? That's impossible.
I do not think that anyone will condemn Koeman for the defeat in the classic - Real looks much stronger than Barcelona, which is in a difficult situation through no fault of the coach. But if Barcelona lose their next Champions League game, then that could really be a reason for retirement. Dynamo Kiev is not a strong opponent at the moment and a defeat will cost Barcelona an entry to the playoffs.