Penny got a good number of throws in before he felt something. He threw a normal pregame warmup, took some long-toss, and simulated an inning in the bullpen at Coors Field with a hitter standing at the plate but not swinging. When Penny came in from the bullpen to the main mound on the field and got ready to face hitters, he felt something and stopped throwing.
Penny was not available to speak with reporters, but it seems that the issue on Tuesday was less his back, where he initially injured himself, and more in his right triceps.
"[After the bullpen work], he took a break, came out to the field, started warming up to face some hitters, and he felt some tightness in his triceps," pitching coach Dave Duncan said. "So as a precaution we shut him down."
The exact severity of the injury will remain unknown until at least Wednesday. Penny left Coors Field and was scheduled to return to St. Louis on Tuesday night. He will be examined by the team's orthopedic staff on Wednesday.
"I'd characterize it as a setback," said assistant general manager John Abbamondi. "Hopefully, a modest one, but I don't know. We're going to send him back to St. Louis. He's going to fly back tonight and be evaluated by our doctors there tomorrow."
Penny was on track to make a Minor League rehabilitation start on Sunday if Tuesday's throw had gone well. He has not pitched in a game since May 21.
"It's a big negative," manager Tony La Russa said.