The Rossoneri midfielder is expected to miss the next six months after falling to the ground awkwardly midway through the first half of his side's victory in Turin.
The Dutch defensive midfielder went down unopposed in an attempt to win the ball back in the 22nd minute, and tests later revealed that he suffered a serious injury.
"AC Milan announce that during the match between Torino and AC Milan, Nigel de Jong sustained a subcutaneous rupture of his left Achilles tendon," Milan wrote in a statement on the club's official website.
In response to the news that the 28-year-old would miss the remainder of the campaign, his coach Massimiliano Allegri replied, "It's a terrible shame about De Jong, who will be out for the rest of the season, and of course all the lads dedicate the win to him.
"We'll see what to do about De Jong's role in January, but we already have Sulley Muntari, Massimo Ambrosini and Mathieu Flamini."
De Jong joined Milan in the summer from Manchester City for an initial 4.5 million-euro fee and has made 15 appearances for the San Siro outfit, scoring once in the process.