No. Defensive teams are expected to anticipate probable field goal attempts, whether that is 4th down, or a down during a time-critical situation. If they need to sub, they need to do so in the same time frame as any offensive subs.
This direction from ‘above’ came about as a direct result of defensive teams abusing the substitution rule, by taking their time to make substitutions, to have the clock expire while they were making their subs, denying the offense the opportunity to kick a field goal.
In the same amount of time as it takes the offense to run in their FG team, the defense is expected to make any substitutions they may want to make.