It's already been clear enough to see for over a year now that he has declined physically to some extent, but not exactly in the way you're describing. He still has that energy, explosiveness and speed, but what has noticeably diminished in my opinion is his capacity to employ these attributes with the same frequency as pre-injury (ie. stamina). That said, I doubt you'll see a noticeable difference in the 2nd half of the season as he's capable of overcoming this issue by adapting his game as he's been doing so far.
Re: last season's performances, he started off excellently (hat-trick at the Mestalla, last-minute heroics v Sevilla, hat-trick v Ajax) until getting injured against Almería at the end of September, after which he came back immediately but was struggling (with his match fitness, performances and "stat" output) for a good month until he got injured again and was out until January (he failed to score in 5 league apps, but remarkably did score 3 times against AC Milan in that period). From then on until the end of March he was brilliant in some games (Getafe & Levante in the cup, Sevilla (A), Man City (H), Real Madrid (A), most notably), woeful in others (Levante (A), Valladolid (A), Espanyol (A) and Betis (H) especially) but mostly just plain "invisible"/ineffective in the final 2 months of the season (both legs v Atlético in the CL, apart from one moment of playmaking brilliance that Iniesta couldn't finish; the Copa final v RM, and basically every other game after his hat-trick at the Bernabéu).
It was strange to see him going through games with so little influence in the build-up, so little energy (even if he wasn't pressing like under Guardiola before he was almost catatonic now), not even that many "flashes", partially due to the accumulation of injuries throughout 2013 (he had to leave the pitch injured 5(!) times that year) and subsequent physical decline and partially due to Tata Martino's tactical scheme less involving Messi (he both made and received the least amount of passes per 90min compared to his previous three seasons and also took the lowest % of his team's shots).
This season he's shot out of the starting blocks once again and has been a lot more involved in the build-up, both on and off the ball (making more runs and making himself available more often), and with pressing and defensive work (averaging nearly 1 successful tackle per game). More positives: an increased number of Key Passes per game (the highest average in his career so far). Negatives: losing his clinical touch (both shooting accuracy and conversion rate are down by a lot) and having a far worse success rate at dribbling than anytime before (which is likely related to said physical decline).
It'll be interesting to see how the addition of Suárez will affect Messi: will he become even more of a "provider" or will he get more space to score himself? Or both?