Villanueva, who is set to hit free agency at the end of the season, allowed seven runs in just 2 1/3 innings during his last start against the Rays.
The next day, manager John Farrell indicated that Villanueva was feeling more discomfort than usual, but it wasn't entirely surprising at this point in the season.
"He's back to normal," Farrell said. "He is ready to start [Wednesday] and on tap for that. But as we've stated other times, too, this time of year guys are dealing with some general fatigue and that's no different in this case."
Villanueva has two starts remaining in what has been an impressive season. The veteran righty is 7-6 with a 3.88 ERA, but has excelled the most since being moved into the rotation in late June.