Question

Average acceleration of soccer ball

Answer 1

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 .