Still even if he is a champion or even he is known as undisputed fighter he still needed to prepare for his upcoming fights and not be overconfident on what kind of opponent he is going to fight with. It is better to be prepared than being overconfident if Magsayo is really looking to win this fight. Which in fact, Garry Russell is not also a small fighter that he need to be confident, I believe this fight will be a difficult fight for both fighters but I am also rooting to Mark Magsayo to win this fight.
It's okay to be confident but never underestimate an opponent whether you are a champion or not. It's necessary to ensure that proper training is done since this is a championship fight, and the loser here will lose the chance to have bigger fights in the future. For Magsayo as a challenger, he should give everything he has for this once-in-a-lifetime opportunity.
Yes, this would serve as stepping stone for Magsayo and for sure he will not waste this opportunity, but you are right, we can't underestimate Russell capabilities, he's not a champ buying the belt, he's here to prove that he is really deserving for this title, both fighters are in need of this fight for their future, they need to compete and entertain all the viewers and fans, good fight will serve them more exciting offers..