 He made his debut in a 0–0 draw with bitter rivals Man United at Anfield in a First Division fixture on New Year's Day 1920; he had to wait until 15 April 1922 to open his account for the "Reds", again it was at Anfield and the opponents were Cardiff, his 73rd-minute strike was the fourth goal of the 5–1 defeat of the "Bluebirds".McNab, a wing-half, struggled to break into the starting line-up during his first two seasons at Liverpool; he made just three appearances in this spell.It wasn't until ten games into the 1921–22 campaign that McNab nailed a permanent role in manager David Ashworth's plans but only after Jack Bamber and Francis Checkland had started the first fixtures; he took his chance well and only missed four of the remaining matches in the first of the back-to-back title winning seasons.