There aren't any legitimate candidates from the pool of position players. Mark Trumbo has amassed 26 homers, but that has come along with a .256 batting average and sub-.300 OBP. Desmond Jennings and Brett Lawrie have been killing the ball since their call-ups, but neither will finish with even half a season of games played.
Over on the pitchers side, things aren't much more competitive. Earlier this season it seemed like nobody would be able to match the performance of Michael Pineda; he took a sub 3.00 ERA into the month of July, and put up fantastic strikeout numbers along the way. His strikeouts haven't dropped off—he still sports a 9.23 K/9—but his ERA has risen all the way up to 3.74, and he owns only a 9-9 record thanks to poor run support. He also stands a chance of being shut down after another start or two in order to limit his innings.
Nova's ratios don't compare to Pineda's. He loses in ERA, 3.74 to 3.89, and in WHIP, 1.09 to 1.34. The strikeouts aren't close either: Pineda almost doubles him, 163 to 85. However, this isn't going to matter when the award is handed out, because Nova's record is fantastic. He is 15-4, with wins in eight straight starts and eleven straight decisions. The Yankees haven't lost a game that Nova started since June 3rd. With one more win he'll have the most wins for a Yankees rookie since Ron Guidry in 1977.
If he gets to 17, he'll match the win totals of Justin Verlander and CC Sabathia in their rookie seasons. Verlander won Rookie of the Year, Sabathia didn't—he lost to Ichiro, whose .351 average led the league in batting. While we're considering all possibilities, if Nova gets to 18 victories, he would be the first AL rookie since 1976 to do so.
With the way he's been pitching lately, it doesn't make sense to bet against him.