Pars boss Stevie Crawford - who spent time at Tynecastle as assistant to former manager Robbie Neilson - confirmed that the 18-year-old was undergoing treatment after suffering damage to the discs in his spine.
McDonald joined the Championship side on a season-long loan deal in August but is yet to make his bow for the Fifers.
• READ MORE - Big interview: Ian Murray MP on battle to save Hearts, Hibs fans’ emails to Lithuania and his all-nightersCrawford said: "It's a frustrating situation but we've come to an agreement with Hearts that he won't come back to us until they deem him fit.
"He's their player, so we just have to make sure he's 100 per cent ready to come back.
"There's no timescale on that, but he'll report daily at Hearts until he's fully fit."
Meanwhile, Craig Levein has revealed he hopes to have Peter Haring available later this month.
The Hearts boss told the Evening News: "He is out on the grass running. If he doesn’t have any setbacks then we should see him relatively soon.
"There isn’t a definitive recovery period for this kind of injury. He is back running and doing ball work, so things are progressing well.”