I live in Scotland, and as all that is shown are Rangers or Celtic ( a pub will even change channel mid spurs game, to put on either of their games), as a result I have seen Cuellar play a lot this season. He's a good player, very tough tackling, but he lacks the pace or awareness to make it at the level we wish to be at in the premiership. I was at the Ranger - Lyon CL game (due to some fancy courtesy tickets) and Benzema tore him apart, and it is that kind of player, rather than the likes of Scott McDonald, that he will have to face in the premiership.
Benzema did not tear him apart. I believe he made one mistake in that match, which led to the second goal. Either way, it's one game. You can't just mention one defeat and blame Cuellar for it. Hutton played in that game too, as it happens.
Here's another game for you. Lyon 0-3 Rangers. Like I said, one game is hardly proof of anything. I could mention better sides than Rangers who have lost 3-0 to teams like Lyon.
He's proven himself plenty of times. Rangers' defensive record has been tremendous. Osasuna, essentially a side in a perpetual battle against relegation, made the semi-finals of the UEFA Cup last year. Cuellar was a key player in that run. I'm not sure what makes you think he lacks pace and awareness. He is nothing if not constantly aware, and he does have plenty of pace. It seems to be a common misconception in here that Cuellar is slow.