Johnston is out of the lineup for Wednesday's game against the Cubs.
He'll join fellow left-handed hitters Edouard Julien, Sterlin Thompson and Brett Sullivan on the bench while the Rockies face off against lefty Shota Imanaga in the penultimate game of the series. Johnston had started in each of Colorado's last 19 games, slashing .293/.333/.373 with three stolen bases, 10 runs and seven RBI over that stretch.