How to create a fireball effect with After Effects?
You might want to check out a class of effects known as "particle generators."

Trapcode Particular is a nice 3rd party option, but costs a bit of money.

AE has some built-in particle generator effects, too. Check out CC Particle World, CC Particle Systems II, and Particle Playground.

This tutorial here covers using CC Particle World to create fire. You could adapt that example to make a fireball instead of a wall of fire.

Particle Generator effects often have an source position property that you would need to match to the position of the football. This could be done with motion tracking, or simply keyframe it 'by hand' if the ball throw segment is fast enough.

Note that CC Particle World and CC Particle Systems II are very similar, but the first is a 3D effect while the second is a 2D effect. For shots where the ball goes from closeup to far away, the 3D variant would be helpful, but otherwise you might be able to get away with the simpler 2D variant.