I think Neville's comments (if accurately reported) leave him open to a possible legal action for defamation. Posing his comments as a question makes no difference in legal terms. He has essentially accused city's accountants and directors of false accounting. The fact he hasn't named anyone also makes no difference (in fact it could open him up to multiple actions from numerous parties)
If the club decided to sue Neville then the burden would be on Nevillle to prove what he has said is true. Good luck with that one.
I have always thought City should pick their legal battles carefully. But personally I think this would be one worth fighting, not least because Neville may be considering running as Mayor of Greater Manchester. If his comments are accurate then he's a liar and needs to be challenged in public on what he has said.