I think your choice of words is useful. It's not that he is wrong, but that he's done wrong. The week before the season, he/his agent has/have gone to the Sun and vented. There's a lot of truth in what he says, but it looks shitty, particularly as some of what he says could be viewed as dissing a lot of his teammates.
He signed a new contract a year ago that ties him until 2021. Any suggestion that he's underpaid should be viewed with that in mind. There's absolutely no way on Earth that he's on 30Kpw. There's some suggestion that because he's been injured for so long that he might have missed out on a lot of bonuses, which is possible.
It all comes down to the same thing again. There are five English clubs with revenue that is at least 100M more than ours. We have a fair few players who are as good as or better than their counterparts at those clubs. If the market rate is set by Man U, Man C, Chelsea, Barca etc then sure, our players are underpaid. If you want the best footballers to be at the "best" clubs and paid "the going rate", then bully for you, just don't whinge about the CL or FFP etc etc. You just want different things from football than me.
Just because you can't compete financially with the top clubs doesn't mean you can't spend money at all