illegal substitution on the defense??

[MI Official]

If he came BACK in before the down it would be Illegal Substitution, dead ball. After the snap, same as before (did he or didnt he influence the play) IS or IP.

I believe that no player shall leave and enter or enter and leave after being replaced during the same dead ball situation unless a period ends, a fould is accepted or a timeout occurs.

Had this situation a few years back, when B thought a was going to kick for a try but when they lined up for a non-kicking try, all the subs ran back off. we ruled it IS live ball but was subsequently informed by our association that its a dead ball foul as soon as the replaced players come back on before the snap.