Harper was pushed in a marking contest and landed awkwardly while playing for Werribee against Coburg on Saturday.
North Melbourne director of medical services Steve Saunders said a specialist would review Harper's injury within the next day.
"He'll be in a moon boot for the next week … worst case would be four weeks, but we're hoping to get him back well before then, perhaps in two weeks if all goes well," Saunders said.
The classy forward was groomed this pre-season for an expanded role in North's midfield after playing 37 games in his first two seasons.
But Harper was dropped after averaging just five possessions in this year's first three rounds and has not been able to win a senior recall since, despite being named as an emergency in several games.
North reaffirmed its faith in Harper recently, re-signing him to the end of 2015.
Nick Bowen covers North Melbourne news for AFL.com.au.Follow him on Twitter: @AFL_Nick
