I hate to break it to you but the 70-200 mm will not have enough reach, unless you don't mind running up and down the sidelines, which from these shots looks like you did not do.
Backs of heads don't work well. For that reason I always shot soccer from the end line, near the goal.
That way I could shoot the players as they moved towards me so I got their faces and expressions without having to move much, if any.
One team approaches in the first half to attack the goal at the end of the field you're set-up at, the other team attacks the same goal in the second half.
I used a 150-500 mm zoom on a monopod to shoot the other end of the field (think goalie), on a second body an 80-200 mm f/2.8 for mid-field shots, and on a 3rd body a 25-85 mm for action around the goal.
It looks like you got low, which is good. It not only helps with the backgrounds, it also makes the players seem a bit bigger and more powerful.
I sat on a walkstool with one knee on the ground:
Walkstool Comfort Stool
I always set up a 10 x 10 pop up canopy so I had somewhere to retreat to. The pop up also served as my store, where I had samples on display and album books of 4x6 prints people could look through. I sold 4x6's for $12 each, or 4 for $40. 20x30 faux magazine cover prints were $300 each.