By losing two attacking players who are the spearhead of the Barcelona team's strength, this will make it difficult for them to get a win. Hopefully, Robert Lewandowski and Dembele still have the opportunity to contribute against Real Madrid. Barcelona lost their last match against Almeria, so when they play against Real Madrid, they will also lose badly because they will definitely be beaten until they are no longer in the mood to compete.
Real Madrid most likely lose La Liga to Barcelona so they will more likely concentrate to beat Barcelona in Coppa del Rey. They also have Champions League to fight and it's reasonable to give up in La Liga and reserve energy of their players for more achievable targets like Coppa del Rey and Champions League.
Barcelona have an opposite situation, they have to secure their first position in La Liga to win it at the end. Europa League ended for them days ago in England. Now, Coppa del Rey is second title they can take in this season and I don't think they will easily award a win to Real Madrid but with effects from injuries and a bad sentiment after a defeat in Old Trafford, very likely that Real Madrid will be a winner. They can fail in La Liga but they won't want to lose directly against Barcelona and fail in one more competition.
Dembele and Lewandowski are very important for Barcelona attacking system and I am sure Xavi has a headache before the match in Coppa del Rey.