After appearing five times in the pre–season friendly matches, it wasn't until on 26 August 2014 for Sadler made his Rotherham United debut in the second round of the League Cup, which saw them lost 1–0 against Swansea City. However, this turns out to be his only appearance in the League Cup, before returning to Crawley in September on loan. He was linked with a loan move to Notts County and Crewe Alexandra. Sadler then had a run in the first team, which lead to extending his loan spell for another month. After extending his loan spell, Sadler continued to remain in the first team before returning to his parent club on 13 November 2015.