I would imagine it would be a rather easy fix to add a quick logic check for users. The check would go as follows:
If the user has heals & they just entered a battle with < max health (or < 90% of max health to leave it open for people who want to go in with 1 or 2 points of damage), a pop-up should appear confirming the intent to do so and giving the option to consume a heal before entering the battle. Alternatively (or in addition to this), one could also have a setting that auto-heals the active planeswalker if they have a potion and the setting is toggled (call it “auto-heal”).
On the rare case that there could be people who do this regularly (and so the pop-up could become annoying), one could also add a check-box on the pop-up that says “do not ask again” or some such thing.
I think this would be very helpful as one does occasionally (if absentmindedly) hit “enter” when one has %10 of their health. The inevitable auto-quit, though probably nice for the opponents’s win count, is ultimately not fun for both users (whose intent, one would assume, is to play the game).
This kind of setup would also provide a nice opportunity for teaching new users how to use health potions, how health potions recover, etc.–a detail that other users have mentioned in the forums.
Thanks! 