There's no single word to translate underdog. This is the beauty of languages. Often you can't simply translate a single word. You have to consider the whole sentence and rephrase it in ways more commonly expressed in the target language. As a result, when asking for translation
处于劣势的一方 and 居于下风者 are the better translation if picking among those.
弱者 means someone who's weak or inferior. 失敗者, 失敗的一方 means loser or someone who's defeated. These are wrong translations with different meaning.
The underdog team scored two surprise goals.
This is from a dictionary. But it's more common to say [Name of the team]隊出人意料地進了兩球。 Because 'underdog' is sort of implied in '出人意料地'。
As for what @scaaahu suggested, 黑馬 could be used as the translation, BUT only if the team has won. And it'll only be used in sentences like: 他/某某隊是這次比賽的黑馬。 黑馬 is of the contest or competition. The underdog team cannot be translated to 黑馬隊, which sounds weird.
I think 黑馬 means the same as dark horse in English. From oxford dictionary,
dark horse: a candidate or competitor about whom little is known but who
unexpectedly wins or succeeds
and from 教育部國語辭典,
Notice dark horse and 黑馬 don't have the meaning about the little chance of winning that underdog conveys. 黑馬 may implied it, but not necessarily.
菜鳥 means exactly newbie. But when using just 菜 as an adjective, it can mean bad at something and/or describing a newbie.
As a native Chinese speaker, I've never heard of 魚腩部隊, and I'd say it's not just me. Don't use that. People probably won't understand.