I spoke to Matthews a little while after he left City & he told me that Dave Jones said that he wasn't wanted so he had no choice other than to leave. Maybe leaving Celtic was a mistake as I remember him being quoted in the press at the time as saying that he needed to play a higher standard of football to further his international ambitions - that has certainly backfired as he has gone backwards rather than improve his chances.
Matthews has the ability & as far as I can recall has never had a reputation as problematic so not quite sure what is happening with him across the bridge (it might be down to Lee Johnson as most of his players appear to have given up on him). I would be happy to see Matthews back here as I am pretty sure that whatever the issues are (if any) Warnock would sort them out in no time.