You are correct, when the helmet comes off, he is not a substitute, he is a player. However, the rule requires that he go out 1 play. For that one play, he is not an eligible substitute. If you read the substitution rule, during any dead ball period, any number of eligible substitutes may enter, etc. The player whose helmet came off in the previous play is not an eligible substitute. If he enters during the one down in which he must sit out, he is an illegal substitute, hence the illegal substitution foul if observed before the snap. If he participates, he is illegally participating.