Thinking about it, it's probably not a coincidence that Messi went to Italy to meet up with his dietitian just now. The last time he really looked full of energy was against City at home, and since then he has felt the decline in energy and is now doing something about it.
I think they planned out a complete diet for him at the start of the season that for sure helped to make him leaner, quicker and more agile again - areas where he was lacking last season. But, it could be that as the intensity has been ramping up towards the end of the season, with two games a week being the norm, the diet now needs some adjusting as it perhaps gives him too little to work with in regards to calories. He does look too thin now, so it could be that some of the tiredness he has shown during recent games is partly down to him not eating enough to compensate for the increased activity.
If true, that would be awesome news, because it means that he will soon become more energetic and strong just by adjusting his diet, and not necessarily just by resting more. I mean, if a part of why he's feeling tired is down to not eating right, then by all means fix that.
Time for him to eat a pizza - doctors order!