The answer to this question depends on the amount of bone available and also the position of the sinus floor. There are instances where, replacing the tooth might be to invasive therefore best left not replaced. Second opinion can always be useful.
Not sure if you're talking about a 2nd or rd molar here, but I think you are referring to a second, so let's assume that. You may not have to replace it, but what I think the dentist is trying to tell you is that it's not a simple replacement and may not be worth the effort, in your case. You can function perfectly well without it, but if you feel that you'd rather have it back then you could look into options like an implant.