[kdf5]

Illegal formation, 7-2-1 . . . After the ball is marked ready for play, each player of A who participated in the previous down and each substitute for A must have been, momentarily, between the 9-yard marks, before the snap.

PENALTY: Illegal formation (Arts. 1, 2, 3) – (S19);

It can't be illegal substitution. Which one of these did he violate?

3-7 Substitutions

ART. 1 . . . Between downs any number of eligible substitutes may replace players. Replaced players shall leave the field immediately.
ART. 2 . . . A player, replaced player or a substitute is required to leave the field at the side on which his team box is located and go directly to his team box.
ART. 3 . . . During the same dead-ball interval, no substitute shall become a player and then withdraw and no player shall withdraw and re-enter as a substitute unless a penalty is accepted, a dead-ball foul occurs, there is a charged time-out or the period ends.
ART. 4 . . . During a down a replaced player or substitute who attempts unsuccessfully to leave the field and who does not participate in or affect the play, constitutes an illegal substitution.
NOTE: Participation by a replaced player or substitute is illegal participation as in 9-6.
ART. 5 . . . An entering substitute shall be on his team’s side of the neutral zone when the ball is snapped or free kicked.
ART. 6 . . . During a down, a replaced player or substitute who enters the field, but does not participate, constitutes illegal substitution.

PENALTY: Illegal substitution (Arts. 1, 2, 3, 4, 5, 6) – (S22); (Arts. 1, 2, 3) dead-ball foul – (S7-22); (Arts. 4, 5) live-ball foul – 5 yards; (Art. 6) non-player foul – 5 yards (S22)

Possibly Illegal Participation:

9-6-4d...d. To use a player, replaced player, substitute, coach, trainer or other attendant in a substitution or pretended substitution to deceive opponents at or immediately before the snap or free kick.