The Black Cats, who will be based in Derby between the two games, have tasted defeat in three of their four friendly outings after a tough week on the training field concluded in Saturday’s 2-0 loss against Swedish champions Helsingborgs.
And Cuellar knows Sunderland must show more as the pre-season campaign draws to a close, if they are to be in shape for their first league outing at Arsenal in 12 days time.
Cuellar told the Echo: “The next two games are important because we have been training for a long time.
“We know the mistakes we can’t make and we need to treat these two like they are proper league games.
“We have to improve at keeping hold of the ball. But we’re not making excuses, we’re working hard to get fitter and fitter. We had a lot of chances on Saturday and we were maybe unlucky that we didn’t score a goal.
“But we were a little bit open on the counter-attack and they got a couple of goals. The main thing is to carry on working hard and be ready for the 18th.
“They scored a good goal with a strong shot and then in less than five minutes, they scored another one on the counter-attack. We have to learn from that to correct the mistakes for the next games.
“We are still working to improve our concentration and fitness. Last week was hard but that’s what you expect. It’s pre-season and you have to work hard to do your best.”
Sole summer signing Cuellar completed his first 90 minutes of pre-season in Scandinavia after missing the previous friendly against Hartlepool with a thigh strain sustained during the South Korean tour.
“I feel fine. It was my first 90 minutes on Saturday and I felt fit and good,” added the Spanish centre-half.
“I was a bit tired in the end, but nothing serious. I knew there was something wrong at the Peace Cup, but I knew it wasn’t bad and after a week I was fine.”