The way you constructed this question is a little bit confusing, but let's talk about the main word there "underdog" in sports betting this refers to the weaker team or you can say that it's the team that the odds are not in favour of to win. Underdog playing simply means betting on the weaker side that's what it's all about. Most bettors prefer to bet on the favorites because the chances of winning is high compared to the underdogs
This is not the true definition of underdogs my friend. Two big teams can be playing and one is the weakest, does that the weakest among them is an underdog? The answer is no. The typical definition of underdogs are the opposite of the big teams, and by big team, these are teams that have less performance over the years finishing in the bottom almost every season. It can also be considered as teams that is always defeated most times in the comeptition. You said a small team is underdog while big team is favorite, it's not always like in this match. I'm sure people know how big is Manchester United and how high their market value, compared to Nottingham Forrest who're just a small team.
But Nottingham Forrest played better and at the same time Manchester United were in bad perform, this make Nottingham Forrest become favorite due to stronger than Manchester United at that time.
