The 30-year-old has won every club trophy in Spain with Barcelona and has been crowned the Ballon d'Or winner five times in his career so far.
On the international stage, though, Messi has not been as successful, losing three Copa America finals, as well as a World Cup final.
In his upcoming book, which is being serialised by Viva, Sampaoli wrote: "Messi has a revolver put to his head called the World Cup and if he doesn't win it, he's shot and killed.
"As a result, he can't enjoy his talent. What I find is that the negativity surrounding international football damages Messi. At this moment, I feel that I'm coaching the best player in history. This is a guy who has stayed as the best in the world for 10 years."
Messi is expected to play against Spain in Tuesday's friendly after recovering from a minor muscular injury.