Torrealba was examined by the team medical staff, and it was determined he had a small meniscus tear in that knee.
"He experienced some discomfort after [Wednesday's] game that we were aware of," manager Clint Hurdle said. "He tried to address it [Thursday], but it was a situation where it became more problematic."
Chris Iannetta started in place of Torrealba and went 2-for-3 with a double and an RBI in the loss to the D-backs. Because Iannetta was the only other catcher on the 25-man roster, the Rockies called up Adam Melhuse from Triple-A Colorado Springs and optioned outfielder Cory Sullivan to the same club.
Melhuse was en route to Coors Field during the game. In 29 games with the Sky Sox, the 36-year-old Melhuse was hitting .317 with three home runs and 14 RBIs.
"It's the first time I've ever done six innings with only one catcher available," Hurdle said. "It's a little different feeling when you're on the bench. You just hope nothing happens to Iannetta so you don't have to go paper, rock, scissors to pick another catcher."
Hurdle said the Rockies have decided to keep Torrealba off the disabled list and will monitor how his knee feels over the next few days.
Jeff Birnbaum is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.