Like, seven games.
Sanchez began his sustained assault on American League pitching on August 10, and by season's end had reached a number of home run milestones earlier than any player in MLB history. Despite playing only a third of his team's games, Sanchez claimed a share of the AL rookie home run lead (with Texas' Nomar Mazar), revived a moribund Yankees season, and staked a claim to ROY honors.
Sanchez, of course, must contend with Detroit's Michael Fulmer, who distinguished himself with a fine 4.9 pWAR (eighth in the league, despite starting only 26 games) and 135 OPS+ over 159 innings.
It should be noted that some of Fulmer's peripherals suggest an element of luck was at play for the hirsute rookie: Opponents hit .231 against the right-hander, suppressed by an excellent .270 BABIP. Both marks are well below league benchmarks (.255 avg; .300 BABIP). But his fielding-independent pitching (FIP) was a somewhat pedestrian 3.76 (Fulmer's SO rate was slightly below average, while his BB and HR-allowed percentages were slightly better than the league average). Fulmer was very good, but he was also helped by a very good infield defense (second baseman Ian Kinsler, catcher James McCann and SS Jose Iglesias were all above-average defenders).
It must also be said that the league eventually solved the puzzle presented by Fulmer. After a blazing first half, Fulmer's record on July 6 stood at 9-2, with a superb 2.11 ERA. He limped to the finish in the second half, going 2-5 with a 3.94 ERA (with an attendant dampening of all peripherals).
As for Sanchez, his abbreviated season doesn't really lend itself to analysis; the sample size is just too small. Essentially, he had an extraordinary hot streak that happened to coincide with the beginning of his career. He was clearly the best rookie story of the year, but it's not clear that he was the best rookie player of the year.
The uneasy pick for AL ROY is Michael Fulmer.
National League Rookie of the Year
There's much less intrigue relating to the NL award. As much as we'd like to play the contrarian, there's no argument to be made against Los Angeles Dodgers shortstop Corey Seager as the league's best rookie. His .308/.365/.512/137 OPS+/6.1WAR line makes him the easy pick in the National League. Unlike Gary Sanchez, it took a little time for the power to arrive (in this case, all of a month). Once it did, Seager immediately became one of the best players in the league. He should place among the top-3 on the MVP ballot.
Honorable mention to the Nationals' Trea Turner (.342/.370/.567/144 OPS+/3.5 WAR), who was Seager's equal in all things but playing time.
Up Next: Cy Young, MVP