Kolek was optioned to Triple-A El Paso on Sunday, AJ Cassavell of MLB.com reports.
Kolek will head to El Paso after posting a disappointing 6.23 ERA and 1.38 WHIP with 20 punchouts over 26 innings in his last five starts. The cold stretch came on the heels of back-to-back scoreless outings for the right-hander, and he will now look to reset at Triple-A. The move made room for Kyle Hart to join the major-league roster, and Hart is expected to operate as the team's bulk reliever behind opener David Morgan against the Rangers on Sunday.