I doubt we can sign a La Liga forward for Barça B. Even if we'd promise him to play for the first team this would be just too obvious and RFEF wouldn't let us register him.
And if we'd get someone from Segunda/Segunda B (it would probably be someone who's not playing much for his current team on loan because no team would sell/loan us a player they need without a possibility to replace him) to cover for Hiroki's injury he wouldn't have enough quality to help the first team. What signing a possible replacement for Hiroki would allow us to do is maybe calling up someone like Collado or even Konrad for the first team more often. But if we'll eventually need to call up 2 or 3 B team forwards for the first team Setien will do this in any case and let Pimienta deal with his squad selections however he knows (by possibly calling up further Juvenil or even Cadete players). Hopefully our injury situation won't worsen though. It's bad enough as it is already.
BTW, I was just thinking yesterday that this new Matheus Pereira guy we got from Juve in the Alejandro Marques deal is training mostly with the first team since joining us eventhough he can't play (he's a non-EU and we're sparing that last non-EU spot for Araujo in case we'll need him; which could happen as soon as this week) when Barça B would actually also need him. Unless there are some bureaucratic/registration problems preventing us from temporarily playing him for Barça B this doesn't make sense to me. Maybe Setien wants to see what we have in him during trainings but would still be better for him to train with/get to know his Barça B team-mates to be ready to play with them when he can.
Thinking more about this there is another possible reason. Maybe we must buy Pereira after his loan expires if he'll play certain number of games for Barça B and we don't want to do this so it's just our way to save some money.
Probably smart from a financial standpoint but a disaster from a sporting viewpoint having a healthy player at a position of need and still not playing him for non-sporting reasons. He's 99% not good enough for the first team but could at least help out Barça B while he's here.