In such a situation, you can STOP worrying and focus on doing your best to move ahead. And that is: Assuming you really want to go to the US for a PhD, honestly and credibly explain why there's no letter from the current supervisor, and leave the rest to the ad com's discretion/decision. Usually, people who are on ad coms are fairly seasoned folks who are familiar with all sorts of legitimate and verifiably true problems that applicants run into. Assuming that the rest of your applications is strong, you just have to take chances, do your best and hope for the best. What have you got to lose? Good luck. oohi ashu