Sigurdsson has been on loan at Rubin Kazan this season, but that deal has been cut short after a permanent switch to Rostov was agreed. The Iceland defender joined Fulham from Krasnodar in 2016 and made 18 appearances, the last of which came in April.
Brighton have been linked with a swoop on League One Sunderland for the signature of goalkeeper Jason Steele.
Southampton remain in the hunt for Red Bull Salzburg's Amadou Haidara and are reportedly readying a £15million offer.
Aston Villa are reportedly weighing up a summer move for out-of-favour Huddersfield defender Michael Hefele.
Southampton are lining up a summer bid for Huddersfield Town midfielder Aaron Mooy, reports claim.