Real Madrid midfielder Modric told Sky Sports News that his teammates must not "get ahead" of themselves following their defeat of the two-time champions.
"Let's not be euphoric or get ahead of ourselves," the 32-year-old said. "Of course this win will boost our confidence for the next game, and we have shown we can create opportunities, but let's keep our feet firmly on the ground.
"Our team effort throughout [was the key] and particularly in the second half when we had more possession. In the first half when Argentina had the ball we cut off [Lionel] Messi and stopped him getting the ball.
"He is the most dangerous player. We created three clear chances which we missed. But in the second half it opened up and we deserved to win."
Croatia's final Group D game is against Iceland on Tuesday.