Following a pair of rough outings, the Cubs optioned rookie starter Caleb Kilian to Triple-A Iowa on Tuesday.
Right-hander Mark Leiter Jr. was recalled from Iowa in a corresponding move.
Kilian, after an impressive big-league debut June 4, has been derailed by command issues since the Cubs recalled him from Iowa last week.
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He walked five in four innings Wednesday vs. the Padres, surrendering five runs while also allowing five hits and hitting a batter.
He allowed seven runs (five earned) Monday, walking five in 2 1/3 innings against the Padres while throwing just half of his 66 pitches for strikes.
The Cubs recalled Kilian, their top pitching prospect, from Iowa amid a series of injuries to their starting rotation. Wade Miley, Marcus Stroman and Drew Smyly are all on the injured list.
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Leiter Jr., who's made three starts for the Cubs this season, and Alec Mills are potential candidates to take what would have been Kilian's next turn in the rotation.
Mills has pitched out of the bullpen since making his season debut earlier this month but went 4 1/3 innings in relief of Kilian Monday.