Steve Clarke is hoping to keep Glenn Murray at Reading after his loan deal expired following the 2-1 win over Norwich.
QPR boss Harry Redknapp admitted he needs to strengthen his squad after his side's 0-0 draw with Crystal Palace.
The former Bolton full-back has completed a medical on Wearside and will officially join the Black Cats' squad when the winter transfer window opens.
Manager Gus Poyet said: "I can confirm Marcos is going to be with us for the rest of the season on loan from Fiorentina, which is something very important.
"I was looking for a player in that position on the left side and I hope he will help us and everybody will like him.
"I am delighted that everything is agreed."