It was the split-finger fastball that helped Masahiro Tanaka become a star in his native Japan and then jump to the United States and a $155 million contract with the Yankees in 2014.

At its best, the pitch darts sharply inward and down against right-handed batters. Its drastic late movement has left batters whiffing a third of the time they have swung at it — managing a measly .195 batting average against Tanaka’s splitter from 2014 through last season.

But like many puzzled pitchers around the majors these days, Tanaka has not been able to get his trademark pitch to behave quite the way it used to. He has clearly struggled this season — he was on pace for a career-high walk rate and earned run average (4.78) entering his start against the Baltimore Orioles on Monday.

Tanaka did well early against the Orioles and took a 6-1 lead into the sixth inning, when he allowed four more runs and departed with one out. The Yankees ended up with a 9-6 win even though Tanaka allowed 10 hits, walked two batters and pushed his E.R.A. to 4.93.