Cuellar missed two months of action with a hamstring problem, but he has featured on the bench for the Black Cats' last three matches.
The former Aston Villa centre-back is hoping that he will start a match for the club in the near future, but he has acknowledged that he faces competition to regain his place.
The 31-year-old told Chronicle Live: " I understand the manager has a tough decision to make when it comes to centre-halves because it is the one area of the squad where he has plenty of options and I have no problem with that.
"As well as John and Titus and myself, there is also Kader Mangane and Matt Kilgallon but I don't mind the competition for places, I am just glad to be back.
"I am feeling good, feeling fit and for the last few games, I have been happy just to be back on the bench and ready to be called on if needed."
The Spanish defender signed for Sunderland last summer.