The 27-year-old, who was a key component at Spurs between 2008 and 2012, has struggled after switching to the La Liga giants last summer.
Real are yet to see the best from Modric this campaign, despite spending ?33m on securing his signature from White Hart Lane.
The Croatian has made 26 appearances for Jose Mourinho's side, but his national team boss Stimac - who played for Derby County and West Ham during his playing career - has insisted Modric will improve.
"I said at the start of the season that he wouldn't be at his best until the end of February," Stimac told Spanish paper Marca.
"It was to be expected when you consider that he had to get into top physical shape which, without a pre-season, he didn't have.
"Now, at the end of the month, you'll see an improvement. He's grown stronger from playing matches and training."
Stimac claims that his star player is adapting to a different style of football at club level in Spain, and deserves patience from the Madridistas.
"You have to remember that he is adapting to a different kind of football than he played in England," insisted Stimac.
"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."