Don't see the issue with what Pep has said. Given the comments about him on here by others over the season, I'd probably be considered one of Nunes' biggest fans but I said exactly the same thing about Nunes in midfield ages ago. Pep doesn't trust his mind and his first touch to play him in midfield.
I do think its something he could work on and improve drastically if given the opportunity though. if he concentrates for 90 minutes his problems wont be there. It's a mentality thing and I don't believe that's something a player cannot sort out. Doesn't seem like pep is going to give him the opportunity though.
Nunes can be a great fullback for us imo. he's got absolutely everything pep wants from a fullback he just needs the experience there and to stay switched on. If he's happy to play predominately in that position I don't see him going anywhere.
We've had delph, zinchenko, Rico Lewis, gvardiol, Ake and now O'Reilly as fullbacks for pep despite none of them being fullbacks and most of them have done well for us and we've been very successful with them. Nunes has got more ideal attributes than any of those players for fullback.