The 18-year-old midfielder, who joined the Nerazzurri from Dinamo Zagreb in January, has been in impressive form so far in Italy.
"I have great belief in the lad," Stramaccioni told reporters. "I really wanted him here and his talent is there for all to see.
"Mateo does need to be managed carefully, however. He's 18 years old and he had cramp at one point of the game [against Tottenham Hotspur] on Thursday.
"He played something like 70 or 75 balls and only got five or six wrong - that's incredible. He's a player to be managed carefully for the moment and a certain star for the future."
Kovacic has made three appearances in Serie A and four in the Europa League for Inter.