I think this wouldn't be a big problem since the youngsters like Edgar should only play Cup games. We can give our starters their needed breaks with the youngster in the squad.
I don't mean for next year, seeing they would still be playing with the B team. The issue is they'll have to be promoted sometime soon enough. If they buy 2 CBs, next season they'll have Pique, Bartra, CB 1, CB 2, which is the maximum a team needs, and is perfect for that season. They'll also need to buy another DM (Mascherano won't stay to play backup DM ONLY), not a problem for that season either.
The next seasons however, ideally you'd want to promote both Ié and Samper, but there won't be too much space in the squad, and they'll have to be loaned out, which is not always the best idea for La Masia players.
If however they only buy ONE CB (someone with a lot of experience, who can also teach things to the younger ones like Bartra and later Ié), Mascherano is guaranteed a to have a lot more minutes in both CB and DM next season, so there are essentially 4 CB and 2 DMs (and if somehow both Busquets and Mascherano are injured, they can call Samper up), which are the ideal numbers for the squad. And the next seasons Mascherano can leave and Ié and Samper can take him places at CB and DM respectively.