1. Accept the challenge;2. Reduce your pressure and risk

1. If you don't, your boss will be disappointed, for you don't have the gut to take on any challenge.

2.Pressure vs Risk: what if your boss finds someone whom you don't like, or he doesn't like you?

A better appraoch: tell your boss and your former supervisor that if he wants to come back anytime, you will give this position back to him and work under him

3. If you make a mistake or can not live up to your boss expection, look from a different perspective. Somebody might screw up more than you do.

Bottom line: you are the best suitable candidate for this positon, next to your former supervisor.