If we desperately need a goal it's better to have Messi, Neymar & Suarez on the pitch. Why would Luis Enrique turn to Pedro?
If Neymar is being woeful with his finishing (Or any other forward sans Messi, really) they should be subbed. Obviously, Lucho wouldn't turn to Pedro, because of how poor a player Pedro is. If we had someone decent like, say, a more developed Munir/Sandro on the bench, they would be a good option to bring on. We actually saw a glimpse of that against Villareal, bringing Sandro on to more or less save the day.