There are only four quick-slots for potions, and going into your inventory to fill them back up takes too long in combat. My recommendation: when you use a potion from your quick-slot, if another potion of the same type (health or mana) is in your inventory, it is automatically drawn into the quick-slot as a replacement.

Actually, there's an even better solution: get rid of the quick slots and just have a health potion button and a mana potion button on the main screen: tap one to drink the appropriate type of potion from your inventory.  You could put a number on the button to indicate how many potions of that type you have left in your inventory. If you're completely out of either type of potion, the corresponding button is greyed out. You could also color-code the number (or the border of the button) to provide an additional indication that you're low (red=1 left, orange=2 left, yellow=3-4 left, green=5 or more left). You could even give players a way to control exactly which potion they drink: a single-tap or double-tap causes you to drink the potion that will restore the highest number of points, while holding down the button will cause you to drink the potion that will restore the lowest number of points. That way if you're in serious trouble you can get the most help quickly, but if you're not super-worried you can take the extra second to only use a weaker potion.