McDonald has netted three times in eight games this season but has faced criticism from some quarters of the media for a perceived lack of fitness.
However, the Aussie hit back at the weight jibes by revealing a lean torso after scoring in Saturday’s 4-0 drubbing of Hamilton.
“How can people give him stick when he scored 31 goals last time and this season has barely started?" Robson told the club's official website.
"He's a strong wee guy and he'll bang loads of goals in again.”