It seems that Xavi will use De Jong as replacement for Lewandowski if he cannot recover and play in the match against Real Madrid but De Jong is also playing after recovering from his injury so Xavi has to really consider everything so that nothing bad happens to his players.
However on the other hand some of the latest news also states that Lewandowski can recover on time and can even take part in El Clasico and this is good news even though it is not necessarily true.

But despite all the rumors that have been circulating it seems that Xavi knows more about the Barcelona squad so he can have better plan to be able to perform optimally in El Clasico.