Instead, Arroyo has an inflamed bursa sac that caused an MRI to be misread, according to Fox Sports, an injury for which he could return to throwing in seven to 10 days.
Arroyo said Thursday he was waiting for the results of another MRI but the rotator cuff in his right shoulder was "significantly torn," and his injury was not a torn labrum as had been reported.
Arroyo said he would opt against surgery in any event.
"It's either rehab or retire," he told MASN on Thursday.
The 39-year-old Arroyo, a 15-year veteran with a 145-131 record and a 4.19 ERA, averaged 207 innings per season from 2004 to 2013 before undergoing Tommy John surgery on his elbow in 2014.
A 2006 National League All-Star with the Cincinnati Reds, Arroyo signed a minor league deal with the Nationals in January.
Arroyo was scheduled to pitch Wednesday's exhibition game but was scratched with shoulder soreness. Manager Dusty Baker said Arroyo had felt soreness in the shoulder since last starting on March 10.