Sinclair signed a new four-year deal on 15 August 2007. He was given squad number 17 for the new season, and made his first appearance as a late substitute in the 2007 FA Community Shield against Manchester United. On 25 September, he scored his first senior goal for the club in a 4–0 win against Hull City in the League Cup. He was picked in the starting eleven against Leicester City in the fourth round of the League Cup; he was involved in Chelsea's first two goals and his shot was turned onto the post by Leicester's goalkeeper.