Caretaker Pompey boss Guy Whittingham wanted to bring Racon in on transfer deadline day but could not complete the deal, and is now waiting for the loan window to open later this week.
"We tried to get Therry as a free but the deal couldn't be done by the end of transfer deadline day," Whittingham told The News.
"We were hoping he was going to cancel his contract at Millwall and that went on until the last minute.
"Now it has to be a loan because we are still interested in him.
"Obviously things aren't certain yet but hopefully we will be bringing him in on Friday."