Barca have sold Aubameyang to Chelsea in about 15 million deal,and gave Marcos Alonso as part of the deal.It is pretty fair because they wanted Aubameyang by all means since they are not getting the number of goals they required.He is brought to be a messiah to the club.So far,it's only Sterling that has been doing so much on the field,contributing and scoring goals as well.
I think 15 million is a cheap price for a great striker like Aubameyang. While Marcos Alonso seems not really needed for Barcelona since they already have Jordi Alba. If Xavi wants a substitute player for the left-back position, it should be better to sign a young player. Although Marcos Alonso is a strong left-back but he is already 31 years old, he won't play for a long time with his best form.