Yes, but they are going to do that on 3rd and 15 with any team, even against Oregon. The refs will not signal ready and start the clock if substitutions are being made between downs.
If the offense doesn't make a substitution, they do indeed put the ball in play and signal ready. The only way the referees hold up play is if the offense makes a substitution. In that case, the defense is allowed to make one as well. Otherwise, the defense runs players out to sub at their own peril if the offense decides to snap the ball.