Modric's side went top of Group D as they beat Nigeria in Kalingrad after Argentina were held to a 1-1 draw against Iceland in the other game in their group.
Nigeria conceeded an own goal by Oghenekaro Etebo before the Real Madrid midfielder secured his nation's win with a composed penalty.
Reflecting on the game, the Croatian captain said "I believe that on the wings of this win we will up our game and continue our journey in the way that we planned before coming here.
"Argentina is going to be a very difficult match against maybe the favourites in the group,
"But this win will boost our confidence and as we go along, we will raise our game - but to get three points against them will be tough."
Croatia will face Argentina on Thursday and will look to secure their progression into the last 16 with a win over the South Americans.