It will see the Power utility miss the rest of the season after the club confirmed a break in both bones.
Stewart was tackled by Adelaide's Jared Petrenko in the first quarter of the traditional local derby against the Crows.
The Power utility was assisted from the field and replaced for the rest of the game by Port's nominated substitute Sam Colquhoun.
"That will mean he's out for the season," said Port Adelaide senior coach Ken Hinkley.
"It's disappointing for Stewy but that's this brutal game we play."
portadelaidefc.com.au will provide a comprehensive injury update on Monday morning.
