Liddell entered the youth team on his arrival in 1938. Before he established himself in the side, Liddell had been frustrated at the frequent rotation of players until trainer Albert Shelley advised him to be patient. An injury incurred against Blackburn Rovers threatened to end his career prematurely. He had struck his knee against concrete near the corner flag after being challenged while running with the ball. Doctors informed him of the extent of tissue damage, causing Liddell anxiety about his future. He remained in Blackburn for two weeks, living in the family home of a colleague, before returning to Liverpool to continue his recovery. The Second World War precluded an official debut for Liddell, as the Football Association suspended competitive football from the abandoned 1939–1940 season and instituted a regional system in its place. Until the cessation of hostilities in 1945, Liddell guested for various domestic sides when unavailable for Liverpool and represented select teams, including a Football Association XI and Scottish Services XI. Liddell was capped eight times for Scotland during the war, scoring on his debut in a 5–4 win over England in 1942.