Mattock suffered ligament damage to his left ankle in a 1–0 League Cup win over Stockport County on 12 August 2008. Fortunately, there was no fracture, meaning he would be out for several weeks instead of months. On 28 August, three days before the close of the transfer window, Leicester rejected seven-figure offers from two unnamed clubs for Mattock. He scored his first and only goal for Leicester in a 2–0 win over Yeovil Town on 19 January 2009. Leicester went on to regain promotion to the Championship by finishing the season as league champions.