Starlin Castro is off to the best start of his career. He's currently hitting .335/.372/.514, 18%/6% K/BB, 143 wRC+. His impressive performance one of the reasons why the Yankees are off to a surprising 26-16 start. Unfortunately, I'm not so sure it's for real.
Castro has been a talented, frustrating major leaguer for years. He debuted for the Cubs at the tender age of 20, and put up 3.0 bWAR seasons at 21 and 22. He rightfully earned the reputation of a potential superstar breakout candidate. However, the breakout never came; Castro was never as good over a full season as he was at 22. He was a below average player from 2015-2016. Entering this season, Castro looked like a placeholder until Gleyer Torres was ready to play 2nd. Now, he's a good bet to represent the American League in the 2017 All Star Game.
However, it's not all that clear if much has changed for Castro. His strikeout and walk rates are in line with his career norms. Almost nothing - groundball/flyball proportions, hard/soft contact, etc - has changed in his batted ball profile. His mediocre exit velocity hasn't changed much:
His batted ball profile yields a xwoba of .323 - almost identical to his .322 Steamer projection entering this season.
So what's happening? Castro is pretty clearly getting very lucky on batted balls. His BABIP is 0.381, up from right around .300 over the past two seasons. The Starlin Castro we knew from before this season is very likely to be the Starlin Castro going forward.
Gleyber Torres was just promoted to Triple-A after knocking around Double-A pitchers. He could very well force his way on to the major league roster by the end of June. The conventional wisdom seems to hold that Chase Headley's job is in danger, even though Headley has clearly been the better player over the last two seasons. The Yankees should still consider replacing Starlin Castro at second instead.