Nelson, a rookie who has started the last 12 games, also was limited in practice on Thursday.
If Nelson doesn't play against the Steelers, second-year safety Jamaal Fudge will likely make his first career start.
Nelson's injury could be another setback for a defense that lost three-time Pro Bowl defensive tackle Marcus Stroud (ankle) for the season last week. Jacksonville also is playing without linebacker Mike Peterson (hand) and defensive end Reggie Hayward (groin).
Defensive end Jeremy Mincey (hamstring) also missed practice on Friday and was listed as questionable. Defensive end Paul Spicer (shoulder) and cornerback Rashean Mathis (groin) were probable.
Copyright 2007 by The Associated Press