He is behind Harden and Westbrook in just about every advanced stat metric. I believe he is still the best player, but is sandbagging a bit, keeping his teammates involved and making them a better team in preparation for the playoffs.  Basically, he could be the MVP if that were his goal, but right now he isn't playing for the MVP and therefore isn't as high on the stats as he needs to be to pass those two up.
It is a two horse race and rightfully so.  MVP doesn't mean best potential player, but the best player as they performed this season.  Right now he hasn't been the best player during the 2016-17 regular season.