Per Ryan Divish of the Seattle Times, Hernandez was diagnosed with shoulder bursitis, and the expectation is he will be able to return within three to four weeks.
Hernandez was one of the most reliable aces in baseball during his peak seasons with the Mariners. He's been selected to six All-Star Games, led the American League in ERA twice and won the AL Cy Young Award after a 2010 campaign where he posted a 2.27 ERA.
His numbers have faded in recent years as he's watched his velocity drop, however, and the 31-year-old starter has also become an injury concern. The right-hander failed to reach 30 starts last year for the first time since 2005, and this injury was initially deemed to be a case of "dead arm" due to right shoulder inflammation.
The latest setback will further test Seattle's pitching depth, which has been pushed to the limit during the season due to a steady stream of ailments, including the elbow injury of projected No. 2 starter Drew Smyly. Chase De Jong is getting a spot start Sunday against the Cleveland Indians.
Ultimately, Hernandez's days as a top-end starting pitcher could be a thing of the past, but he can still be a valuable piece of the staff. The Mariners will hope the injury issues don't become a consistent trend because his lucrative contract keeps him with the organization through at least 2019.