"He felt some discomfort in the elbow throwing and down his forearm," Uhlman Jr. said. "Did the right thing coming out. We will meet with him and have the doctors take a look and examine him and see where we go."
The 31-year-old right-hander hasn't pitched in the big leagues since 2013. He was throwing one-inning simulated games before he began his rehab assignment.
"It's concerning, especially for a guy like Josh, who in my time here has worked as hard or harder than anybody to come back from this type of surgery and injury. From the training room, to the weight room to the mound -- he has put in a lot of effort. So you always feel bad," Uhlman said. "Until we see him and until the doctors take a look at him, I can't give you a whole lot more. It's especially disappointing for him because we were hoping that he could get in 25 pitches and go on to the next outing on Monday."
It's been a long road for Johnson, who rehabbed from his April 24, 2014, surgery, with the hopes of being able to make it back for the 2015 season But he had a setback in April, when he developed tightness in the nerves in his neck and his throwing program was halted, essentially forcing him to start over.
"I don't think you expect setbacks," Uhlman said. "You want things to go as smoothly as possible, but obviously there are going to be setbacks. Once he gets on track you want him to continue to progress and to this point Josh just hasn't been able to do that."
Before heading out for his rehab assignment, Johnson said he felt good and was hopeful he would be able to compete again.
"Being here, showing up every day, working hard, [being able to pitch again] would make it kind of easier to go through," Johnson said on Tuesday. "It's been tough, it's been a long road, but things happen for a reason. I don't know why right now, but it's all part of it."
• First baseman Yonder Alonso, who is day to day with back tightness, starting playing catch Friday and began taking dry swings. Uhlman said Alonso is feeling better and making progress.
• Relievers Brandon Maurer and Shawn Kelley are both continuing the treatment phase of their recovery.
Maurer has been on the disabled list since Aug. 10 with right-shoulder inflammation and will meet with Dr. Hoenecke on Friday. Maurer threw from 60 feet on Wednesday and felt discomfort.
Kelley is suffering from a strained right forearm that he injured when throwing a warm-up pitch in Tuesday's game against the Rangers.