It's not a soccer (football) specific word, but underdog ALMOST fits. From the OED:
underdog (noun) - a person, team, country, etc. that is thought to be in a weaker position than others and therefore not likely to be successful, win a competition, etc
Before the game we were definitely the underdogs.
In politics, he was a champion of the underdog (= always fought for the rights of weaker people).
Note: underdog has a connotation of being weak. You asked for a word that has a connotation of aggressive, but weak/ineffective. Underdog is the closest word I can think of.