There is a very real chance. Every MMA fight has the opportunity for an upset. Nate Diaz is one of my favorite fighters, but I'm not going to be delusional and not think that McGregor doesn't have a very good shot and winning the fight.
I'm just kind of curious as to where it goes. If McGregor wins, does Nate get a rematch for Diaz-McGregor III? Is McGregor's title going to have no one fighting for it while the Diaz-McGregor fights go on? If they are just going to fight each other, what's the point of dropping much weight if they are both going to cut for it?