Warthin was unclear on whether she needed to be carried out on a stretcher or was able to walk off, but she reported that the park visitor was "in good condition".
"This just seems to be sudden", he said.
Ranchers are concerned the bison could spread a disease called brucellosis to their cattle.
Superintendent Dan Wenk announced last week that he meant to retire March 30, 2019, after being offered a transfer he didn't want. The startled animal head-butted her off the path.
At least eight other senior executives are being reassigned.
"This situation is unique", he said. He said he had disagreements with Zinke about the number of bison at the park but had believed those to be resolved.
"There is a pretty strong, nearly militaristic hierarchy that seems to be in place that's kind of tinged by the expectation of loyalty associated with it", he said.
But Wenk said NPS Deputy Director P. Daniel Smith quickly rejected his March plan and gave him an ultimatum: Move to Washington to lead the National Capital Region in August, or get out. He said he's heard Zinke talk about employees not being loyal to the Interior Department. In 2017, there was only one incident at Yellowstone of a visitor being injured by a bison while 2015 saw five such incidents.
And Wenk said he was not anxious about getting reprimanded for speaking out.
The only dispute he's had with U.S. Interior Secretary Ryan Zinke, who oversees the park service, was whether the park has too many bison, Wenk said.
Wenk said he doesn't know if he'll take the job or retire early.
Yellowstone, which occupies the northwestern corner of Wyoming and parts of Idaho and Montana, is home to the nation's last large herd of wild, purebred bison.
Zinke's office didn't return Yellowstone Public Radio's request for comment.