Taking the thumb a problem?!

Beebee

My pediatrician saw today that my LO is taking her thumb and she said not to get her used to it ASAP. She said LO will get overbite issues and will be very hard to change the habit later on.

She suggested to give her a rag or a dummy! But I think the dummy is just as bad. She has found her thumb to sooth herself. What can I do?