Messi wasn't good in the three season defining games against Atletico, not in some random game during the season. Then, he didn't have a single great game during the WC against a strong team. He started very well in the final but then faded pretty badly.
IMO, he's not the same player anymore. He lost some of his energy, explosiveness and speed. However, he was the best footballer ever before that, therefore, he's still one of the best in the world and can decide games on his own, particularly against avarage teams. But he's declining physically and this will become clear in the second half of the season when he will have played some 35 games or so. Watch this space.
Of course, I could be completely wrong. I'll be the first to admit it and I will be happy for him and Barca. The longer we watch Messi in his peak as a player, the better for football in general. Wish him to avoid any injuries and prove me wrong.