If Munir gets invited up to preseason, I wouldn't be surprised if Lucho actually did. Within a few months of becoming appointed at Celta, he moulded Rafinha to be his star man in central midfield. By September, he was already accelerating the progress of Santi Mina (Celta's own extremely young striker prospect) and gave him a surprisingly high amount of game time. He had gotten Hugo Mallo back to the great form he showed way back before the ACL injury he suffered against Ronaldo. Even Fontas became a decent looking player under him. Also, Lucho gave some non-star youngsters like David Costas and Jonny Castro a lot of chances and made them into better players.
Competition is obviously higher at Barca than at Celta Vigo but there is a route that allows it to happen. First off, some of our better attacking players (Messi/Neymar) will probably have long WC runs so have extended vacations and miss a chunk of preseason. Munir will get to train with the first team and maybe even play in some friendlies, getting exposure to Lucho's system right from the start. Then, assuming that Suarez comes and Alexis leaves which is looking more likely each day, we'll have one starter from our front line banned for a couple of months. If Munir gets off to a flying start with the B team (which is more likely than not) then he'll have the room to get regular call-ups to train and maybe even make his debut. Throughout those months we'll have throwaway matches in the CL and CDR where the B team will feature and all of this in totality gives Munir a chance to make an impression on Enrique and get some attention. After that he'll be favored to come up during injury cases, during rotation spells, etc. just to accumulate as much exposure and as many minutes as possible.