UFC twitter says this...
Due to Iaquinta not making the 155-pound championship limit, the UFC lightweight title will only be eligible to be won by Nurmagomedov. Should Iaquinta win, he will not be awarded the title, but if Nurmagomedov wins, he will be crowned the undisputed champion.
Is there some rule that says they can't just award him the vacant belt if he beats Khabib?