As Andy stated, put your camera into aperature priority, set it for f8 and try for a minimum shutter speed of 750 by changing the ISO setting. Then turn off IS. Make sure that you have the focus set to AI Servo with only center focus point active. If you set your camera to Program or green or sports mode, it will pick the biggest aperature (lowest fstop) possible while keeping other parameters reasonable. So most likely, it is picking f5.6 for your setup. For most consumer grade lenses, you are safest or most likely to produce good results when you add two stops to the biggest aperature for your lens.
After you shoot and you are reviewing, check your focus point - until you get used to tracking and keeping an erratically moving object in the focus point, you'll see that a lot of out of focus shots are because you just missed having the subject in the focus point.
You didn't state what your dissatisfaction with the pictures was. The zoom is known to be soft on the long end so were your shots mostly in the 250-300 range? Were the players faces in shadow (this is where a L lens helps)? I see a lot of photographers not pay any attention to their positioning in relation to the sun and they wonder why their shots didn't come out well. All it would have taken was a walk to the other side of the field or another end and they would have increased their success rate tremendously.
There are times when you will have to boost your ISO to 800 or 1600 to get the shutter speed you need. Then you will need something like NeatImage or Noise Ninja to take the noise out. It is a sad fact of digital photography - especially when dealing with lenses that don't have "pro" quality optics (there are some reasonably priced lenses with good optics) that some skill in post processing is required to render good results. Something people don't realize is that film processing incorporates a lot of computerized processing that "fix" picures automatically. With digital, you are now doing the automated process that has been incorporated into processing film into prints.
Good luck.