Hernandez went just five innings and 85 pitches in his last outing on Friday against the White Sox after "tweaking" his back during the third inning. The issue seemed minor at the time, but manager Eric Wedge took him out when his fastball started getting up in the fifth and he gave up a pair of home runs.
Hernandez's back was tight on Saturday and Sunday and he continued getting treatment.
"He's still feeling it today," Wedge said before Sunday's series finale with the White Sox. "Tomorrow will be a big day to see how he's feeling. He's limited with his activity right now, but we're hopeful he feels better tomorrow."
The Mariners have an off-day Thursday, but with Hernandez scheduled to throw Wednesday, that doesn't affect his status. Rookie Erasmo Ramirez is a potential fill-in candidate in Triple-A Tacoma, but he started on Saturday and threw 103 pitches in a six-inning victory and would be on just three days rest if he threw Wednesday.
Long reliever Hisashi Iwakuma is another spot-starting candidate and has thrown well in relief in two of Seattle's last three games, getting the save in two of them.