"We're going to hold off before we make any announcement on that," manager Brad Ausmus said.
Nathan was scheduled to throw one inning and up to 25 pitches in the Mud Hens' game Wednesday morning against Louisville. He had thrown 10 pitches (seven for strikes) with a groundout and a strikeout when his first-pitch fastball to Louisville's Bryson Smith left him in pain.
"He felt a pop," Ausmus said. "He thought it was possibly scar tissue, tried to throw another pitch, but it didn't go so well."
Nathan threw a warmup pitch but was clearly in pain, and he promptly left the game. Nathan's fastball registered at 88-89 mph, with a few sliders mixed in at 85, according to John Wagner of the Toledo Blade.
Nathan made a brief appearance at Comerica Park before heading out for an examination. He told Wagner and others at Fifth Third Field, "I've never broken my arm in my life, I assume this is what it feels like."
Nathan, who went on the 15-day DL two weeks ago with a right elbow flexor strain, was anticipating Wednesday's appearance being his final step toward a return as soon as Friday. Ausmus hadn't committed to that timetable, preferring to wait and see how Nathan felt.
"It's frustrating," Ausmus said, "because I think he felt like he was close."
Joakim Soria is 5-for-5 in save opportunities in Nathan's absence, creating some suspense as to what the Tigers will do in the ninth inning once Nathan returns. That decision now appears to be on hold while they wait to see the extent of Nathan's injury.
Regardless of the role, the Tigers felt like getting Nathan back was going to be a boost.
"It hurts," Ausmus said. "Like I've said, we're a better bullpen with Joe in it."
A longer-term absence for Nathan could put more of a sense of urgency on getting hard-throwing youngster Bruce Rondon back. He has been on the disabled list since the start of the season with biceps tendinitis, and had his own setback earlier. Rondon threw a brief bullpen session in Lakeland, Fla., on Tuesday, according to Ausmus, and supposedly felt fine. He's expected to throw a full bullpen from there and continue his progression toward facing hitters and a possible rehab stint.