Average acceleration of soccer ball

Average acceleration = (change in speed) / (time for the change).

Apparently, you don't want us to have any of this information.

But an answer is still possible:

You didn't describe the time period that you're considering, so

I get to pick my own time period during the game.

My choice: The time between the kick of a stationary ball and

the instant when it stops rolling and comes to a stop in the grass.

Change in speed = (0 - 0) = 0

Average acceleration = (zero) / (whatever the period of time is)

= zero.