Having made only 10 league appearances, McGrain suffered the first serious set-back of his career on 25 March 1972. He sustained a fractured skull during a match at Brockville against Falkirk after a clash of heads with forward Doug Somner. McGrain, however, made a full recovery and began the following season, 1972–73, as Celtic's first-choice right-back in place of Jim Craig who had left the club at the end of the previous season. McGrain now became a first-team regular and made 30 league appearances that season. He also rose through the ranks at international level, making two appearances for the Scotland Under 23 side in the spring of 1973, before finally making his debut at full international level for Scotland in May 1973 against Wales at Wrexham. McGrain picked up his first winner's medal at the end of the season when Celtic clinched their eighth successive league championship title.