Looks as though this might be the one:

THE INDISPENSABLE MAN

Sometime when you're feeling important,
Sometime when your ego's in bloom,
Sometimes when you take it for granted
You're the best qualified in the room.

Sometimes when you feel that your going
Would leave an unfillable hole,
Just follow these simple instructions
And see how they humble your soul.

Take a bucket and fill it with water,
Put your hand in it up to your wrist,
Pull it out and the hole that's remaining
Is a measure of how you'll be missed.

You can splash all you wish when you enter,
You may stir up the water galore,
But stop, and you'll find that in no time
It looks quiet the same as before.

The moral in this quaint example
Is do just the best that you can,
Be proud of yourself, but remember---
There's no indispensable man.