I was reading though the Stuttgart forums and found out a bit about Sosa.
A few comments were made which I have translated
Sosa is good, no question. But is he such a super super super super super player that is enough for Pep?
As much as I like to see Sosa playing with us, but with his injury history we should better sell him now. Who knows what it will be like in the future. Later he hurts himself and we have neither money nor players
Sounds like he is a good player , needs development to be on our level and is injury prone.