The 23-year-old scored three goals during a spell at Fratton Park last season and had been lined up to rejoin the League One outfit before sustaining the knock.
"We had everything lined up with Sheffield Wednesday. They were more than happy for him to come. He was training there Sunday morning and was due to meet us at the hotel on Sunday night. Then he turned quite sharply in one of the sessions and has done his medial ligament," Portsmouth manager Michael Appleton told reporters.
"I am not sure how bad it is but when I spoke to him on the phone yesterday afternoon after a scan, they think it is going to be around the six-week mark. That pretty much sums up the luck we are having at the minute."
Maguire began his career at Aberdeen.