Sadler made his Walsall debut, in the opening game of the season, in a 1–0 win over Leyton Orient. In the next game against Hartlepool United seven days later, Sadler provided an assist when he was fouled in the penalty box, leading to penalty and was successfully converted by Jon Macken in a 1–1 draw. His impressive display in the first team lead to being praised by Manager Dean Smith, who gave him a nickname as 'Mr Reliable'. Then on 5 November 2011, Sadler provided assist for Jamie Paterson, in a 1–1 draw against Huddersfield Town to end three games without a point. However, in a 1–1 draw against Charlton Athletic on 10 December 2011, Sadler was sent–off after a second bookable offence in a last minutes and missed a match in the FA Cup second round replay against Dagenham & Redbridge, which saw Walsall lost in a penalty–shootout. Despite the suspension, Sadler remained in the first team and then scored his first goal for Walsall on 27 March 2012, in a 3–1 win over Colchester United in what BBC Sport described as "breaking one of football's longer droughts in his 193rd league game (also 222nd in all appearance". Then in the match against Huddersfield Town on 28 April 2012, Sadler provided assist for Florent Cuvelier, in a 1–1 draw, to ensure that the club will be playing in League One next season.