"A contestant that is unlikely to win" is your best choice. Since the female quarterback (Jasmine) is playing for a losing team, conventional wisdom expects the team to continue to do poorly in the league.
Here is another review that will highlight Jasmine's role as a "longshot".
The Harvey Colts used to be one of the strongest teams in the Pop Warner league, but lately things have taken a turn for the worst. Recognizing that his old team needs some fresh blood in order to make a comeback, a former player trains his 11-year-old niece Jasmine to throw the ball like a true gridiron giant. While the powers that be scoff at the idea of a female quarterback, they soon begin to sing a different tune as Jasmine leads the Harvey Colts to the championships and an invigorating shot at redemption.
Because the title of the film is "The Longshots", we can assume that the other members of the team, as well as the coach, are all considered to be contestants who are unlikely to win.
Enjoy the film!
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