The former Tottenham midfielder has struggled to win over Real fans since his summer arrival.
"I said at the start of the season that he wouldn't be at his best until the end of February," Stimac told Marca.
"It was to be expected when you consider that he had to get into top physical shape which, without a pre-season, he did not have.
"Now, at the end of the month, you'll see an improvement. He has grown stronger from playing matches and training.
"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 has been surprised by how fast the game is in Spain."