The 22-year-old suffered the problem when he collided with his post during the midweek defeat to Leyton Orient.
Cisak, who has made made 47 appearances for Latics since joining on a free transfer from Accrington over the summer, had a scan on Wednesday where it was revealed he has fluid around the joint.
"The specialist says the best-case scenario is three to four weeks without having seen the scan," manager Paul Dickov told his club's official website.
"Alex received one hell of a whack. It was a horrible impact with the post and then a Leyton Orient player fell on top of him.
"The specialist was astonished he played on for 45 minutes."