"Until we go find out what's going on, I don't want to speculate on what it is," Nathan said. "But obviously, it was enough for me to take myself out of a game. And anytime somebody walks off the mound is never a good thing."
The 40-year-old Nathan, who had targeted Friday as his return from the disabled list with an elbow strain, was scheduled to throw 25 pitches for Triple-A Toledo as he entered the game in the seventh inning.
However, he clutched his elbow and was checked by trainers after a strike to his third batter and immediately went to the clubhouse after just one warm-up throw.
"It feels like I broke my arm," Nathan said. "I never have broken my arm, but I would assume this is what it feels like."
Nathan said he did feel a pop, but initially "thought it was going to be scar tissue because I did feel something like that."
He returned to Comerica Park later Wednesday for tests, and Tigers manager Brad Ausmus said more were likely to come and that the Tigers were going to "hold off" on an announcement about the injury.
"It hurts," Ausmus said. "We're a better bullpen with Joe in it, no question. So if it's extended, it hurts, but until we know what it is, I really don't want to speculate how long or what the impact is going to be."
According to MLive.com, Nathan threw seven of the 10 pitches for strikes and was throwing his fastball in the 88 mph range during the outing.
"I was real happy [with the pitching during the appearance]. That's why it's scary and disappointing," Nathan said. "Just that one pitch came out of the blue and caught hold of me."
Nathan had said that he didn't have any soreness after a full bullpen session in Detroit on Monday.
"It's just weird," Nathan said. "I feel like we've done what we needed to do to get to this point. I felt great in the bullpen. Even when I was preparing myself, my forearm would always have what I would call normal bruising and tightness, but it was fine. It never restricted me. And today in the bullpen, it felt even better."
"I know I came back quick, but we also listened to my arm," Nathan said. "And my arm has been responding very well. And it responded well today. Until one pitch."
Nathan, in his second season with the Tigers, has 377 career saves, which ranks seventh all-time, over 14 seasons. He struggled in 2014, blowing seven saves and finishing with a 4.81 ERA.
He missed the entire 2010 season after having Tommy John surgery.