The Blues are reported to be targeting former loan defender David Unsworth. The 31-year-old was recently told he is not in new Pompey manager Alain Perrin's plans.
Unsworth has another year left on his contract at Fratton Park and, as we speculated last week, would need to come to a settlement with the Premiership side on the remainder of the two-year deal he signed last summer in order to join the Blues permanently.
Another alternative is the one-time England international moving to Town on a season-long loan with Portsmouth paying the majority of his wages.
Unsworth's versatility - he can play both at left-back and centre-half - is likely to appeal to Royle who is short of experienced defenders.
Royle says he could sign two additions to his backline before the beginning of the season: "Ideally I would like two, but I certainly hope to sign one.
"Having spent a substantial sum on signing Sam Parkin, I imagine any other signings would be Bosman free transfers or season-long loans."
Unsworth spent three months on loan with Town at the end of last season, making 16 appearances and scoring on his debut in the away victory at Sheffield United in February.