Imanaga, 31, strained his left hamstring when he went to cover first base in the sixth inning of Sunday's 4-0 loss at Milwaukee.

Top pitching prospect Cade Horton will be considered for the opening in the rotation, though the Chicago Cubs have not made any decisions on what route they might take.

Chicago's bullpen subsequently allowed a pair of inherited runners to score, impacting Imanaga's line for the worse. The book closed on his afternoon with him surrendering two runs on four hits and a walk. He also struck out four of the 22 batters he faced.