In the 2008–09 season, Sadler started well, appearing for the first three matches to the start of the season until he suffered a lateral knee ligament injury. After missing out five matches, Sadler regained his first team place and then provided assist for Tamás Priskin to score the first goal of the game, in a 3–0 win over Southampton on 18 October 2008. Sadler, once again, sidelined when he suffered an injury that kept him out throughout November. Following his return, Sadler played against his former club, Birmingham City, on 6 December 2008, which saw Watford lost 3–2. Ahead of the match, Sadler stated he hope the club regain back to the Premier League. Sadler eventually fell from grace at Vicarage Road and made his final appearance for Watford on 26 December 2008 in a 2–4 home defeat to Bristol City. During the match, Sadler came under criticism when he conceded two goals before being substituted at half–time. As a result, Sadler was sent to play for the reserve for the remainder of the season and made fifteen appearance.