Coach Pete Carroll said Monday on 710 AM Seattle Sports radio that Penny suffered a broken fibula that will end his season. The running back also injured his tibia.
"It's going to be a haul for him," Carroll said. "This is what they refer to as a tib-fib incident. He broke his fibula, cracked that, but it's the high-ankle sprain element of it that makes it difficult. There's a particular surgery they do to tighten down those bones. It's going to take a number of months, so he's going to miss the season."
Penny will undergo tight-rope surgery Tuesday morning and have his fibula plated to stabilize the ankle, NFL Network Insider Ian Rapoport reported.
The Seahawks back faces a four-month recovery and should be healthy by the start of the 2023 league year, when he will be a free agent.
Penny exited Sunday's game on a cart midway through the third quarter after being gang-tackled on a 6-yard run.
The season-ending injury is the latest setback for the former first-round pick, who has never played a full season in five years with Seattle.
"It's really a heartbreaker for a kid who has really bounced back into the limelight of our program," Carroll said on Monday. "He's been through such a long haul, it's really just a heartbreaking loss for him. He's been doing great and everybody can see it, and we've been so excited for him. He's going to miss the rest of this year. He'll come back, he'll be able to play again and all of that. It's a really clear surgery process and all of that, but it just does take a long time."
Penny played in 10 games in 2021 and caught fire down the stretch. His promising end-of-year play led to a one-year, $5.75 million contract to remain in Seattle after the club had declined Penny's fifth-year option. Penny will be a free agent after the season.
The 26-year-old rushed 57 times for 346 yards and two touchdowns in five games before suffering the injury.