The Spaniard is believed to have suffered the problem after a heavy clash during a Champions League fixture against Bayer Leverkusen in 2015, which damaged the 22-year-old's kidneys.
Saul has claimed that the damage threatened to end his football career, but his "dream" of winning the Champions League with Atletico has seen him play through the pain barrier.
"For the past two years, I've played with an internal catheter," Saul told Mega. "In every training session and game, I was urinating blood. I've had a bad time of it.
"I was playing with my health to wear these colours, to wear a dream. The Champions League is a dream that we all have and we'll fight for many years to make it come true. We'll play with our heart and desire to achieve it."
Saul, who has been linked with a move to Manchester United over the last 12 months, has started 23 of Atletico's 32 La Liga fixtures during the 2016-17 campaign.