With something like 70-200mm you not shooting that far and I don't think there will be that much change required in flash power as player moves closer. So I would stick with 1 setting of manual flash and see how it goes beofre I try to change flash power on the go.
Night time football where ambient light is bad is like shooting in studio. Whether you use 1/60sec ot 1/250sec in studio, the exposure doesn't change. It is controlled by flash power. Now you want to use the highest ss possible while you can still sync flash to the camera. If you choose something like 1/125 or 1/60, you interducing more ambient which you don't want to.
With eTTL camera decide things for you and will change flash output based on what it thinks is right. With manual flash you get same flash output shot after shot.
BTW - I have no exp with night ime football. Just starting myself but I do shoot some with studio lights.
