In March 2005, Mellor was ruled out for the remainder of the 2004–05 season after undergoing surgery on both knees. This robbed the young striker of his chance of European glory as his teammates succeeded in bringing a fifth European cup back to Anfield. Although Mellor played no part in the knock out stages, his contribution to that year's success was massive. Mellor returned to action for the Liverpool reserve side in the 2005–06 season coming on as a substitute in their 2–0 victory at Manchester United in December 2005. He was back on the scoresheet in his first start in a year as Liverpool cruised into the quarter-finals of the Liverpool Senior Cup after a 3–0 win over Runcorn.