My biggest concern regarding his transfer to Barcelona is - what is his supposed role? Whose replacement is he?
If we're planning to stick him on the right wing, then I'm OK with it. Although I'd prefer a left-footed player for that role. Anyway, that would mean he's being brought in as an alternative to Pedro / Sanchez, which I think would probably be an upgrade. If he turns out to be any good in Europe, he'll be fine in that role, even if he doesn't score much. If that's his role, then fine. If he's coming in to replace Villa, then I'm very worried. To me it's a completely different situation. Not in terms of which side of the pitch he's physically positioned at (Pedro and Villa have recently switched wings, anyway), but which player he's being brought in to replace. Right now, Pedro and Alexis are there just to have someone on the wing. We don't really rely on them for much. I could live with Neymar taking up that spot, and possibly improving it if he turns out to be good. Villa, on the other hand, is our goalscorer, our striker, our only real scorer other than Messi. I don't trust Neymar with taking over that role. Not as a direct replacement. We need to have one proven goalscorer next to Messi.
That's why I'm fine with a situation such as: Villa/other striker - Messi - Neymar, but not Neymar - Messi - Pedro/Sanchez. If Villa leaves and Sanchez stays, and we only get Neymar, we're screwed. That would be a downgrade. If both Villa and Sanchez leave and we only get Neymar, we're screwed again. IMHO, Neymar's transfer will only make sense either if Villa stays, or if we get another proven striker along with him. He cannot be Villa's replacement. Only Pedro's / Sanchez's.
Having said that, the most logical thing would be not to buy him. But it's apparently a done deal, so we're all already destined to overanalyze it in our minds.