Jackson is day-to-day with forearm tightness, Justin Toscano of The Atlanta Journal-Constitution reports.
Forearm tightness can sometimes be the first sign of a very serious injury, though the fact that Jackson is merely considered day-to-day may suggest that this isn't one of those cases. While he's resumed throwing, the issue has so far prevented him from pitching in a Grapefruit League game, so it's possible he'll be forced to open the season on the injured list even if his forearm tightness proves to be relatively minor.