McDonald made his NHL debut with the Leafs on October 10, 1973, against the Buffalo Sabres. He assisted on two goals in the game, but also suffered a concussion and required several stitches after landing on his head as a result of a check by Rick Martin. Following the custom of most NHL players at the time, he played the game without wearing a helmet. It was the only time in his career he did so, as he felt that his injury contributed to his early struggles in the NHL. McDonald scored his first NHL goal on October 17 against Michel Larocque of the Montreal Canadiens, but finished the season with only 14 goals and 30 points in 1973–74. His continued inability to score early in the 1974–75 season nearly resulted in a trade. Atlanta Flames general manager Cliff Fletcher sought to take advantage of the Maple Leafs' early disappointment in McDonald and agreed in principle to a trade for the young forward in exchange for Curt Bennett. However, McDonald scored three goals in his following two games causing Toronto to back out of the deal. McDonald recorded a modest improvement over his rookie campaign: 17 goals and 44 points.