#2 Luka Modric - Croatia
The Croatian captain is one of the best midfielders in modern football, but on the international stage, things have never panned out quite right. The golden generation of Croats is slowly coming to pass, and this is his last chance to make that dent.
Argentina's midfield is at their weakest, while the Croatian midfield is one of the strongest in the tournament. If Sampaoli does not play his cards right, and Croatia do, this contest will be over even before it begins, with Modric taking the role of wrecker-in-charge.
He was employed as a central midfielder against Nigeria, but he was a more of a false No.10 for the team and scored against Nigeria. Against Argentina, his long balls will be all the more important.
While his defensive work-rate is not as necessary as it is at Real Madrid, Modric still likes to drop back to marshall the troops. If Messi does find his game, he might have to do that a lot more and thus lose a little sting in the attack. Expect him to make a huge difference though.