I do not have the slightest doubt that Barcelona will not be difficult, the maximum that can happen is a draw in one of the matches, but clearly not a defeat. So the situation in the standings will continue to be difficult. They do not depend heavily on Lewandowski, so it will not be difficult to rebuild the lineup, it is their main advantage
A draw is clearly not Barcelona's goal when facing Espanyol let alone losing. They are only two points ahead of Real Madrid who are in second place, Xavi will not waste the opportunity for an additional three points even though he cannot play Lewandowski in this match. Xavi still has several reliable strikers, the situation will not make it difficult for Barcelona especially when they are playing at Camp Nou, Barcelona will try to do their best to secure their position in the first place.
against Espanyol, Xavi is certainly optimistic that he will get 3 points. Espanyol is not a strong team this season. they are only marginally better than teams in the relegation zone. it shouldn't be a difficult thing for Barca to overcome Espanyol easily.
getting 3 additional points will be good for Barca to increase the difference from Madrid. because now Madrid will continue to follow Barca.
Even though I am also very sure that Barcelona will still win against Espanyol even without Lewandowski, but I feel that victory for Barcelona will never be easy. Because after all, teams ranked below can sometimes provide strong resistance, moreover Espanyol at least has a pretty good record by having several draws when playing against Barcelona.
So yes, I'm sure that at this moment Espanyol can still provide a difficult resistance for Barcelona, but for the final result I still believe that Barcelona will win full points.
Anyway, even though a team is already at the top of the standings, basically they will sometimes still get less than optimal results. But because this is the moment which is their first match after the break, then of course they will be able to play as expected, especially for top-ranked teams.
Unless their match schedule is tight again with the European league schedule, then it is possible that the absence of several key players will have a negative impact on their performance, but at this moment I'm sure that won't happen.