Mate, he's won the Dally M, he's been the Blues' best player since 2007, and he's carved up for Australia.
His club has also run last in the two years he has been injured. Since 2009, the Eels have won 4 from 26 without him (15%). They win more than twice as often with him in the side.
How many other players have the same effect on their team? The teams Greg Inglis has played for over the same period have won 11 from 21 without him (52%). With him in the side they win only slightly more often (62.5%).
What about Thurston? The Cowboys only won five games in 2010, and JT played 17 games that year (winning three of them). Does that mean he was shit too? From 2010 to 2013 (because that's the period I'm using, for argument's sake) the Cowboys won 6 from 18 without Thurston (33%). When Thurston plays they won exactly 50% of games. So he's also not as important to his team as Hayne, though obviously more important then Inglis.
Anyway, importance to one's team isn't the be-all and end-all, but it is a pretty solid indicator of a player's quality.