After signing on a free transfer from Aston Villa in the summer, the 31 year-old established himself as a first-team regular straight away.
However, after suffering a hamstring tear in Sunderland's FA Cup draw at Bolton in early January, the Spaniard has fallen behind John O'Shea and Titus Bramble in the pecking order.
He insists he is far from unhappy with his current situation, and is just happy to be back available again.
"I understand the manager has a tough decision to make when it comes to centre-halves, as there are plenty of options," he told the Sunderland Echo.
"As well as John and Titus, there is also Kader Mangane and Matt Kilgallon, but I don't mind competition for places, I'm just glad to be back."
Cuellar has been on the bench three times since his return to fitness, and credits the club's sunshine break in Dubai for his speedy recovery.
"Being there allowed me to improve more quickly because I could do more and different exercises in the warm conditions," he added.