His fitness is terrible compared to the average Premier League player and that's what will ultimately prevent him from fulfilling his undoubted potential if he doesn't do something about it. I accept that it won't have been easy for him coming back from what was a serious injury but it's pretty disappointing that he's not currently in better shape after his run of games over the Christmas period. He must be doing something wrong whether it be eating the wrong food or not training hard enough.
If he remains injury free and has a full pre-season before the start of the next campaign then he simply has to show a very significant improvement in his conditioning and fitness. Hopefully he will because it would be such a waste if he doesn't.
I think he has shown that as he has got games better fitness levels, the ability to complete more games at the high intensity level he plays at. The problem is/has been that once he plays 5-6 games in a row - at some point it will take a toll on him to where he may tire earlier in games.
This isn't something I attribute to having poor fitness training methods or poor eating habits but, more of a difference in his approach to the game coupled with not having proper down time and pre-season training to achieve fitness levels to the way he plays.
You mentioned average PL player but, the average PL player doesn't play the way Anderson does on the field. Compare Anderson and Carrick, Carrick may cover a lot more ground over the course of the game but, Anderson will tire quicker because of the way he covers that ground - at a lot higher pace intensity.
Try jogging 5 mils and sprinting 5 miles - who do you think is going to tire faster?
Obviously he needs to find a way to be able to play more games on the trot through achieving even higher fitness levels but, also perhaps he needs to find more judicial ways of using his energy on the field. It can also happen with managing his games a bit more where he plays maybe 3-4 games and then sits out 1. If he plays maybe 5-6 games in a row in quick succession but, then sits out 2-3 weeks - then obviously when he gets back on the pitch, the first game back he might not be in peak match fitness form.
Training and eating well will only get you so far in football - there is a reason why they talk about match fitness, games bring bout a whole other level in people.