I hate to be the doom and gloom merchant but for the past 4 years he's played 50 games a season.
By normal standard he's due an injury sometimes soon.
We either have the best physio in the world or we're riding our luck.
No, the reason Leo can play every game is because he's the one player in the system who is told he doesn't have to press and track back. To someone who doesn't know Barcelona and Guardiola it might appear that Leo is being lazy. Off the ball he walks around the pitch, will occasionally look to close down space and very rarely comes back towards our half when we're defending. This is a ploy for several reasons.
1) Messi now plays in the centre. He doesn't have to track back to help out his full back or retreat to make a midfield 5 when on the defensive. Neither does he have to sprint up and down the touchline with Alves like regularity. If Xavi or Puyol had regularly played wide in the latter half of their careers we wouldn't see them play as often as they do. Central and especially central deep positions generally put less strain on a footballers physique, especially in top teams who can retain more possession than the opposition. This is why Busquets will play until he is 65.
2) Thus Messi can conserve energy and use it for high intensity moments.
3) Thus, Messi still has abundant energy even towards the end of the first 45 minutes and towards the end of the second half. When the opposing defence and midfield is tired and is a split second slower to react, or doesn't quite have the energy to close down every space Leo can exploit that.
4) Because of this Messi rarely has to push his body to the limits except for short bursts and thus avoids the normal wear and tear. Factor in his ability to anticipate and avoid the more serious challenges and despite taking plenty of kicks/tugs/pushes he tends to avoid injury (Ujfalusi non-withstanding)
5) This means that Messi can play every game and in the really big games he also has the energy and muscular condition to play high intensity throughout the match (such as against the Milans and Madrids of this world.)
6) Messi is 24. His body is much faster to recover and heal from slight strains, bruising and overwork than a 30+ athlete.
Pep is no fool. And remember this summer Messi is going to be getting a long break anyway whilst his team mates all have the Euro's to play in. If Messi is injured it is unlikely to be muscular in injury, it'll be from a challenge. Fingers crossed that the thug Pepe can't get close enough to him to do any real damage.