The 27-year-old has struggled to hold down a regular place in the Madrid side ever since his £33m summer switch from Premier League side Tottenham Hotspur.
A poll in one Spanish newspaper even voted the midfielder to be the worst arrival in La Liga this season.
However, Modric's national manager Stimac has insisted that he will produce his best football as soon as he has adapted to the Spanish game.
"I said at the start of the season that he wouldn't be at his best until the end of February," Stimac is quoted as saying by Marca. "You have to remember that he is adapting to a different kind of football than he played in England. In Spain, the style is quicker and maybe he needs to get into his stride.
"In England, he was a player who was feared by the opposition. He's got a lot of talent but maybe he's showing a tad too much respect for everyone. He's been surprised by how fast the game is in Spain."
Modric has made 33 appearances this season, although the majority of those have been as a substitute.