It's possible that a small bit of sand could could enough friction to bind the motor and cause it to stop working. It might not happen immediately, but could over time. Best way to check it is to feel the motor temperature after you land. If one is hotter than the others, it's on it's way out and you should replace it.
Blow it out from the bottom, since youre missing the cover anyway. Blow it out from the top. Then tap the top of the motor on a table a few times and then the bottom. If you see any sand come out at all, repeat the process until you dont see any.
Personally, if it were my Mavic, I would replace any of the motors that got sand in them, even if they dont feel gritty any more. But if you are vigilant about checking the motor temperatures by hand after every flight, and also checking the friction every time you fly, you might be ok. If you see any error messages about motor errors or esc errors, land it and have it looked at. It's a warning sign.