Matsui recently underwent an MRI on his back, which showed inflammation but no structural damage, Dennis Lin of The Athletic reports.
Matsui was sent for tests after his back tightened up on him Saturday. The Padres are considering him day-to-day. Matsui struck out the side in an impressive Cactus League debut last week and is in the mix for closing duties in San Diego. He will need to get healthy first, of course, but at the moment, he doesn't sound to be dealing with any sort of major concern.