Yes shoot from lower down but why is everyone suggesting sitting down?
With a 70-200 I thing you'd be better to prowl the side line. This way you are not restricted to just shooting forwards trying to score.
Get a pair of decent knee pads, then knee down.
The lower you get the less of the floor is in the background, which means the background is further away so more out of focus which makes the player stand out better. Kneeling down for 45 minutes at a time will kill your knees and back and make you slightly unstable.
Afew examples for you
This one I was actually sat with my butt on the grass but the boundry is a banking so I am pretty much at the height you would shoot if I was shooting standing.
the ground is up high at about waist height.
This one I am sat on my stool camera is probably 30" above the ground
The ground is below his knees and he stands out a lot better against the distant background of a dark green fence which is not idea really but its a built up area.
Its all about learning, each time you go out and do somthing you will make mistakes like screwing up the white ballence, shooting from a high position and watch for the distracitions in the background (white fence posts) like in the first shot. The 2nd shot is better I was lower, the background wasn't the carpark yes I have those shots, white ballence is better and so on ok a slight distraction of the white bench leg but not like the fence posts.
You will probably never get it right first time out, my first football shots were pants and speaking to Tony (kipax) my next shots were miles better it is all about looking at what you produce and learning from the mistakes. Having someone like Tony who passes on his advice from his experiance is why this forum is great.
I know they are cricket shots and not football but I don't have any football I can link to from work but they show what is being said. When I am shooting football I will shoot one half from the goal line and the other half from the sideline. Goal line can help reduce motion blur if you can't quite get the shutter speeds as there is less sideways movement.