Compare the same (G/A output) with his past 2-3 seasons and you would see the decline (By Messi/Ronaldo standards, not Wayne Rooney standards).
That's a rather simplistic argument. The decline is kinda blown out of proportion because if you scratch the surface, he's scored 0.93 goals per game over the last 3 seasons, and provided 0.40 assists per game - vs 1.12 goals per game, and 0.33 assist per game for the preceding 3 seasons - while having injury issues, and playing a full season under Tata Martino - who didn't have a lot of harmony with the team. So it's by no means a decline of him as a player because the two columns kinda even out, especially when you compare the decreased volume of game he's played in, and some of the tactical inconsistencies the team had.
Another wrinkle to consider is that Messi is not playing as a False 9 anymore (hence he's further away from goal - and not operating in central areas - with direct access to the goal), and his game is more predicated on creating because of the void left by Xavi the puppet master, and sacrificing a bit of his individuality to strike a balance with Suárez and Neymar, and open the game for them. No other player in football is capable of doing that to the standard Messi does - in terms of playmaking, and touch, and his new found ability to 'manipulate' the game, and scoring the number of goals he does - with the frequency he does so.
That's what sets him apart and makes him so unique (like Cruyff in the past, or seasoned Pelé, or perhaps prime Puskás), and that's something no current player will replicate - infact, he's probably a Top 3 passer is football, as things stand - on top of being a lethal, lethal scorer. The mesmerizing things he does with the ball in almost every single game cannot be easily quantified, which is what exasperates Messi vs Ronaldo 'debate' neutrals, and also those who hold Ronaldo in higher regard - because it seems laced with hyperbolic praise. Will leave it at that, instead of indulging in a circular discussion that's been done a million times before, and will rumble on for the foreseeable future.