Check out Oxford Dictionary definition 4.1 for get: Move or come into a specified position, situation, or state. The use of the word get rather than go can also convey a sense of achievement, for example:
How did he manage to get past the guards?
down the left is an adverbial phrase that specifies his route- along the left-hand side of the pitch when moving toward the opposing team's goal.
The term down can be used when moving along a narrow passage, for example:
He saw his chance of glory, and rushed down the lane with Captain Hawley's company of dragoons. - the Last Popular Rebellion