The routine started off with some light tossing into a net and then progressed to throwing on the field. Everything was going OK until Santos felt something when he began some long toss at approximately 120 feet.
"I wouldn't call it a big, big setback," Santos said. "When I got out to 110, stepping on it a little bit, didn't hurt, wasn't painful, just backing off for a day, resting and then throwing again. Good enough distance, playing catch, throwing. It felt fine, the ball was coming out good."
Santos is obviously trying to downplay the injury concerns, but it's certainly not a positive development. He's in a brief no-throw situation and will then restart the throwing process to once again gradually build up his arm.
The hope is that Santos will be able throw a bullpen in approximately one week. The next step would be a couple of rehab appearances in the Minor Leagues, which likely means he won't be able to return for at least a couple of more weeks. If there's another setback along the way, it could be even longer than that.
"Still in the forearm, but it's not bad," Santos said of the spot that is causing him issues. "I'm going to try and push through this and get going soon. I'm still hoping by this weekend, I'd like to get a bullpen in the next week or so."
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.