Incorporate luck in statistical modelling
Use a probabilistic model. For example, if the probability of one team beating the other is $σ(w⋅(team1−team2))$, where $σ(⋅)$ is the logistic function, then there's always a probability that the weaker team will win. 

If you wanted to use a neural network you'd use it to replace the logistic function.

Answer copied from comments.