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Rookie Starlin Castro debuted in early May (2010) as the starting shortstop. However, the club played poorly in the early season, finding themselves 10 games under .500 at the end of June. In addition, long-time ace Carlos Zambrano was pulled from a game against the White Sox on June 25 after a tirade and shoving match with Derrek Lee, and was suspended indefinitely by Jim Hendry, who called the conduct "unacceptable." On August 22, Lou Piniella, who had already announced his retirement at the end of the season, announced that he would leave the Cubs prematurely to take care of his sick mother. Mike Quade took over as the interim manager for the final 37 games of the year. Despite being well out of playoff contention the Cubs went 24–13 under Quade, the best record in baseball during that 37 game stretch, earning Quade to have the interim tag removed on October 19.

When did Starlin Castro debut as the starting shortstop?
early May (2010)