I am not saying the diet has nothing to do with it, but if it is, it would only play a very small part. Jobe has played long enough, is experienced enough, and professional enough, not to undertake something like this without researching and speaking with dietitians/experts first. As stated above there are plenty of Athletes that do just as well, some claim even better on vegan diets.
His levels would be monitored at the club via blood tests, they would know if he is lacking in areas, and adjusting his diet to compensate. So even if there where areas lacking I am sure they would be quickly addressed.
I think Jobe's problems are 2 fold.
The game, and our gameplan has changed dramatically since he last played. All our returning midfield players that lack pace have struggled. Jobe is no exception to this.
The other reason is dead simple he has not played footy for so long that its taken time for his body to adjust to playing the game again. His age is also a factor in this.
Perhaps he is past his best? Perhaps the game itself has also gone past him?
All I know is he has not been given enough time for any of us including himself to answer these questions. If we where half way through the season id be concerned. But as it stands not even worth worrying about.