Saponara arrived 12 months ago from Empoli.
"It really is a good piece of news that Saponara is staying at Milan, I am very happy about it," said Costacurta.
"The boy can become a starter at the Rossoneri, all he needs to do is prove it. We need to give these youngsters a chance without fear."
For the big stories and the banter follow us on Twitter: @tribalfootball