In the 2006–07 season, Sadler began to be first team regular in the left-back position. However, Sadler suffered a groin injury in training, resulting him missing out for four matches. Sadler then made his return to the first team on 30 September 2006, playing 90 minutes, in a 1–1 draw against Leicester City. Sadler played a role in a match against Coventry City on 31 October 2006 when he provided assist for Nicklas Bendtner to score the only goal of the game with a win. After this, Sadler continued to be a first team regular in the left-back position for the remainder of the season until he was dropped from the first team ahead of the match in favour of Stephen Kelly against Barnsley on 9 April 2007. Manager Steve Bruce explained his decision to leave out Sadler of the squad. As a result, Sadler never played again for the remainder of 2006–07 season, as Birmingham City were promoted back to the Premier League. Despite this, Sadler made thirty–six appearance. Nevertheless, Sadler then signed a three–year contract with the club, keeping him until 2010.