De Los Santos pulled his hamstring in late January and won't throw off a mound for three more weeks, Meghan Montemurro of The Athletic reports.
If De Los Santos is indeed back on the mound in three weeks, he'll still have three more weeks before Opening Day, so there's a chance he'll still be able to open the season on the active roster. That's far from guaranteed even if he's healthy, however, as he owns a 5.70 ERA in 30 career big-league innings.