There should be no MMR loss if this happens with a random player. 
If you abandon your partner in Duos before Top 2, you will get an MMR penalty.
If you get abandoned, there should be a notification letting you know you can leave and you will not lose MMR at all.
Note: MMR protection is on only if you are paired randomly. If I join with a friend and they abandon me, my MMR will be impacted.