"I just felt it a little bit, but I'm OK," Rodriguez said. "I just stopped because I felt something like it was tight. It was my second bullpen, so I need to take time until I feel better."
Mills didn't appear to be too worried.
"He threw the ball well, but his back was tight," he said. "It's not anything to be concerned about. That's out of Wandy's mouth, and the trainer said the same thing."
Rodriguez was the Astros' best starting pitcher last year and is a candidate to start on Opening Day. He was 11-11 with a 3.49 ERA in 30 starts in 2011 and has won at least nine games in each of his seven Major League seasons.