So you'd tell Messi in negotiations that he was overrated and 11 of your mates could beat him.
If Messi were to claim that by going it alone, in other words just him, that he'd beat a team of 11 players then it would be reasonable to tell him that he was mistaken - this is no reflection on his individual abilities, nor a suggestion that he is over-rated, merely stating the fact that a single player, on their own, isn't going to beat a team of 11 players, however good that individual's abilities.
But as Jeremy pointed out comparing Messi to the UK isn't appropriate as Messi is the best or second best player in the world - the UK is not one of the top 2 economies in the world, by whatever measure you use.