The 26-year old Japan full-back joined the Nerazzurri permanently from Cesena in the summer after his six-month loan spell at the San Siro last term and has emerged as a key defender for his new club.
"He's one of the players who has improved the most since I've been here and he can still become even better," Stramaccioni told reporters in the lead-up to Monday night's 1-0 defeat at Parma.
"He's humble and he works hard. We're holding on tight to him.
"The individual work that he's doing with Beppe Baresi is also important. If he also becomes good with his left then he'll be even more dangerous."