The Finn's loan spell at S2 has expired, but Blades boss Kevin Blackwell is in talks with the Premier League side in trying to persuade Roy Hodgson, the Fulham manager, to allow their 6 foot 4 inch defender to return to Bramall Lane.
Kevin Blackwell said: "Toni has had a stop-start period with us - through no fault of his own - but he came back in to the side at QPR and did really well for us.
"With Fulham having qualified for the Europa League knockout stages and the FA Cup starting shortly, it might mean that Roy wants to make sure that he has a larger squad available."
Kallio supposedly netted a goal on his Blades debut at Plymouth Argyle earlier this month. James Harper's long range shot at goal found its way into the back of the net, but Kallio did claim he got a vital touch to the ball. Most recently and most impressively, the 31-year-old showed his offensive qualities when he surged into Queens Park Rangers' box on Saturday to pick up the ball and lay it off to Richard Cresswell to score United's equaliser.