It doesn't matter if there's a low chance. It's his job to keep goals out. And Messi didn't give up on the play--he's watching the entire time to see how it develops. You also have to keep in mind that Messi and VVD have completely different roles to play there and that there are different risks to the way they approach those roles. If the ball hits off the ball and lands in a spot where Messi could score but he doesn't, the only cost to the team is one missed scoring opportunity. If VVD gives up on the play and fails to track his man because there's a low chance that Messi will score but Messi does get the ball, then his team has just given up a goal. Like I said above, the margin of error for a CB is incredibly small--one fuck up can completely change a game in a different way than a fuck up by an attacking player. VVD fucked up by not tracking Messi and the result was a second goal, which makes it much more difficult for Pool to move on and also changed the tone of the game with Pool playing with less intensity (I won't say they gave up, but it certainly hurt their morale a lot).