The 2017 season was, by a not small margin, the worst of Masahiro Tanaka’s four seasons with the Yankees. He pitched to a 4.74 ERA (4.34 FIP) with 35 home runs allowed — only Rick Porcello (38), Ariel Miranda (37), and John Lackey (36) allowed more — in 178.1 innings. Tanaka made up for his rough regular season with a marvelous postseason, so that was cool.
The Yankees do not want 2017 Tanaka in 2018. They want 2014-16 Tanaka, the guy who threw 490 innings with a 3.12 ERA (3.53 FIP) and was one of the top starters in the American League.