The Scotland striker, 23, struck three goals in a spell at Fratton Park last season and was due to return to the south coast only to be ruled out for six weeks.
"We had everything lined up with Sheffield Wednesday," revealed Pompey manager Michael Appleton. "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.
"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."