There is no good reason for cider makers, wine makers or mazers (those making mead) to add an airlock during primary fermentation. All you need to do is use some kind of cover - a clean dishtowel or napkin, for example - to keep dirt out. And , since you can anticipate some foaming caused by the production of carbon dioxide - you want to have some headroom in any container you use to ferment. That may mean removing a cup or so of juice from a gallon container... or it may mean using a food grade bucket as your primary or a three gallon carboy rather than a 1 gallon container if you are making a one gallon batch.
All you need to do is wash and re-sanitize your airlock ... and put it away until the gravity of your cider drops to close to 1.005. At that point most of the activity will have slowed and the yeast will no longer be producing quantities of CO2 enough to blanket the surface of the cider (the CO2 inhibits oxidation and oxidation is like rust.. it is not usually a desired outcome )... That is when you add the bung and airlock. Cider ain't beer. You don't need to have the same neuroses as brewers have when you ferment fruit or honey.
There is nothing to worry about in terms of loss of yeast but if you are using dry yeast and the pack costs - what $1.50 or thereabouts there seems to be no great advantage in not using the entire pack - yes, even for 1 gallon - and lots of potential disadvantages in not (if you store the dry yeast in less than optimal conditions and try to use it later then you may find that what is left is not in fact viable so you potentially spoil your next batch for what? the sake of 75 cents?).
Long story short, your cider will be fine.
You are just lucky that there is no solid bits of fruit in the juice because if the solids block the airlock and if the yeast is producing CO2 (half the weight of the sugar is converted into CO2) - then the neck of your carboy will act to rifle the bung and airlock and you can find your ceiling painted with fruit juice.