I shoot with a 7D and 70-200 2.8 IS II frequently. It has been a great combo for basketball and football.
There are a few reasons your photos could be blurry, and it's totally normal to end up with a fair amount of blurry photos when shooting action. The key is knowing why they're blurry and having a good amount of focused images to choose from.
I'd imagine that shooting on a bright, sunny day, you can close down your aperture a good deal to f8 or so. This will help eliminate any sort of depth-of-field blurriness as long as the focus is close. I'm usually always at 2.8 indoors or at night, but action at 2.8 is tricky.
I have the AF set to the center point and just leave it at that. I'll use AI servo most of the time, but sometimes I'll switch to manual focus.
Your shutter speed needs to be pretty high too - usually 1500 0r 2000 or higher depending on several variables. Sometimes you can get lucky with a slower shutter, but it's tricky. If you're following the subject and they stay about the same distance from you, you can drop down the shutter speed to put some motion blur on the background.
Post some photos if you'd like - it's usually easier to tell what's up when we can see photos and camera data (ISO, shutter speed, aperture).