Caulker joined Ronald Koeman's Saints side in the summer, having moved from West London club Queens Park Rangers to the South Coast.
However, the big centre half hasn't featured in Southampton's starting XI since former Celtic defender Virgil van Dijk signed and Sunderland boss Sam Allardyce sees it as a perfect opportunity to bring him to the Black Cats as they try to fight their way out of enduring relegation from the Premier League.
While Caulker is presently on loan at Southampton, if Caulker himself, QPR and Saints all come to an agreement, he would be free to talk to Allardyce about making a potential move to the North East.
Earlier today, Koeman denied wanting to let Caulker leave the club, saying he wanted to keep the former Tottenham Hotspur man at least until the end of the season:
"My plan is that he stays until the end of the season, and then we need to make a decision."
Caulker has been a good addition in terms of squad depth before van Dijk's subsequent move to Southampton. But, he hasn't proved himself to be worthy of a first team place since joining in the summer.
With van Dijk, Jose Fonte, Maya Yoshida and Florin Gardos in the centre-back mix for Southampton, it would be unfair to keep a former prospect to just rot on the bench.