The experienced Spaniard has clocked up more than 120 Premier League appearances with Aston Villa and Sunderland, but is available on a free transfer after being released by the Black Cats at the end of last season.
And the 32-year-old asked to come and train with Rovers as he looks to earn a deal.
Bowyer is still looking to make additions to his squad with former West Ham defender Jordan Spence and veteran goalkeeper Jamie Ashdown also on trial at Ewood Park.
“Jordan’s trained with us on Monday and Tuesday so we’re having a look at him,” said Bowyer.
“We’re not blessed with too many defenders so he’s spending a couple of days with us and playing in a game this week and we’ll take it from there.
“Carlos is another one who trained with us on Monday.
“He asked if he could come and train with us and that’s what he’s done.
“He’ll look to probably continue that. He’s using us in terms of getting fit and training and it presents us with the chance to have a look at him as well.”
Cuellar, who can play at right back or in the centre of defence, came into British football in 2007 when he moved to Rangers from Osasuna.
Following a successful first season at Ibrox he made the switch to Villa Park for £7.8million.
Ashdown, meanwhile, has been short of games since leaving Portsmouth in 2012. The 33-year-old was released by Leeds United in the summer.
Bowyer added that he will not be rushed into further signings, adding that the mentality of any prospective new player was central to his thinking.
But the Rovers chief did admit that he was keen to bring in more bodies ahead of the season opener with Cardiff on Friday week.
“We could do with adding one or two in terms of the strength in depth but I’m no different to everybody else who is saying that,” said Bowyer.
“We’ve only brought two in permanently so we’ve not been really active this summer but throughout the course of the season we might need to improve the numbers.
“That character is really important to us. We don’t just bring anybody in. We check on the character first and foremost and then it’s up to the player to show us what he is capable of.
“Every situation is different. For some players they just want to come and join in training because they’ve something else lined up, and I’m fine with that if we know them and we know what they’re like. Then there are other situations where players are trying to fight for a contract.”
So far this summer Bowyer has signed forwards Chris Brown and Luke Varney on permanent deals, and defender Alex Baptiste on loan.