Greatness is performing when it counts. And although I don't think it's over for Messi, he clearly wasn't one of the greats in the last cup go-round:
World Cup: Messi blows chance for greatness
After Götze had scored, Messi looked exhausted, a ghostly figure stalking the turf in dejection. An injury-time free-kick offered a final shot at redemption; he blasted high and wide. In four years, perhaps there will be another chance, but he will be 31 by then, and the 2005 Under-20 World Cup winning side on which this team is based will be fading. Messi had his chance and, wonderful player though he is, he blew it.
http://www.scotsman.com/sport/football/world-cup-messi-blows-chance-for-greatness-1-3475411