The on-loan Burnley midfielder was told to lose weight by gaffer Eddie May who felt he was looking sluggish.
And after almost a month of intense training and a change in diet, MacDonald says he's a lean and mean fighting machine.
MacDonald, who aims to dump St Johnstone today, said: "The gaffer said I wasn't going to play unless I got fit.
"I lost a bit of weight and I was put on a course to do that.
"He stuck to his word by saying if I lost weight then I would start playing.
"I wouldn't say it was a shock to hear him say it - I knew myself I needed to lose a bit of weight and gain some sharpness.
"Hopefully, the work I have done behind closed doors with Steven Pressley and Paul Green has paid off."