Penny will undergo tests Monday to determine associated damage and to see if surgery is necessary, Rapoport added.
Seahawks head coach Pete Carroll told reporters after Sunday's game that the starting ball-carrier had suffered a "serious" ankle injury.
Penny exited the game on a cart midway through the third quarter after being gang-tackled on a 6-yard run. He had 54 yards on eight carries on the afternoon.
The fifth-year back was replaced in action by rookie Kenneth Walker III, who enjoyed a breakout second half. The second-round back rushed for 88 yards on eight carries, including a 69-yard scamper late in the fourth quarter to briefly give Seattle the lead.
Walker led all Seahawks backs with 26 snaps to Penny's 15 and DeeJay Dallas' 10.
On a one-year, prove-it deal in Seattle, Penny was enjoying a solid season and coming off his best performance of the year to date -- a 17-carry, 151-yard, two-TD afternoon in Detroit. Now his future this campaign and going forward with the Seahawks is up in the air.