In any league I commish failure to submit a complete lineup constitutes an illegal lineup, and that gets you a goose-egg for the week. I'm all for strategy, but the field stays level, and that kinda stuff don't help that happen.
You take a knee with 11 on the field, not 10.
The various leagues I play in have different rules on this, but the most competitive league has a special rule to force complete lineups - starting a player/team on bye results in the worst player in the NFL (not just among active rosters, but the worst possible score) to be substituted that week in place of that player. Submitting an incomplete lineup (not starting someone for a slot, whether through error or intentionally) invokes this same penalty. As this league has defenses that only score points on defensive scores (+3 for a interception return or fumble return for TD), but that scores - 3 pts per TD and -1 per field goal allowed, being forced to take the worst D score of the week is usually devastating to a team's scoring for the week.