Sheffield United boss Chris Wilder admits they're seeking to sign Premier League experience.
Everton pair Mason Holgate and Ben Godfrey have been linked with the Blades.
Wilder said: "We have been linked, and it is vital that we have someone who has played in the division or at a level which is equivalent to the Premier League.
"We want players that make us better as a group and we feel we have done that with Ivo (Grbic) and Ben (Brereton Diaz). We wanted to bring quality in with a few players going out, which they have done. But definitely bringing players in who strengthen the group and push players. There are incredibly important games going on and we are working frantically to improve our group, and hopefully we do that before the window shuts.
"There has to be three things that happen; the player wants to come, we make the correct offer and the selling club, in terms of a loan which is what it is going to be, has to make sure they are covered. Everyone has to look at their group. If someone came in for one of our players, I have to make sure we're covered and not light going into the second part of the season."