The 31-year-old soccer superstar played a little keep-away from his furry friend in his yard Monday ... and it's all fun and cute -- until you realize the dog could end him at any time.

Hulk (appropriately named) is a Dogue de Bordeaux ... and weighs well over 100 pounds. If you're unfamiliar, Messi is pocket-sized, standing at 5'7" and weighing just 159 lbs.

So, neat tricks, Lionel ... but all of Argentina would probably be happier with a nice game of fetch.