MacKinnon, 19, originally injured his foot when he blocked a shot Feb. 22 against the Tampa Bay Lightning, coach Patrick Roy said. X-rays were taken and came back negative. MacKinnon had his first NHL hat trick in that game.
MacKinnon took another shot off his foot Wednesday in a 3-1 win against the Pittsburgh Penguins; he scored the winning goal.
Roy said MacKinnon complained that his foot was bothering him before practicing Thursday. An MRI after practice revealed the fracture.
"It's never good news when you're losing a player like Nate, especially in a key moment in our season," Roy said. "It's been a year like this and we're going to have to win games without him."