"I feel it in the back of the triceps when I extend my arm," Santana said. "I don't think it's serious. I just need a little more time. Maybe nine days will be good enough."
Santana, who last pitched against the Rays on June 11, is eligible to come off the DL on June 27. His DL visit is retroactive to June 12.
"Ervin's just not quite ready," Angels manager Mike Scioscia said. "We'll give Ervin nine or 10 days to get some bullpen [sessions], get some of the inflammation out of his forearm.
"We're a little disappointed. His bullpen wasn't quite where it needed to be. There's always concern. There was concern the first go-round [during Spring Training] when he had a problem with his [elbow] ligament. Where this leads us, you never know."
Where it leads O'Sullivan is back to the mound, where he flourished at AT&T Park with seven strong innings in shutting down the Giants on one earned run. The big right-hander from El Cajon, Calif., near San Diego, yielded five hits and one walk, striking out five men.
"We hope it carries over," Scioscia said. "One thing about Sean is he's shown if he makes his pitches, he can get Major League hitters out."
O'Sullivan, 21, admitted that his head has been spinning a little. In his fourth professional season, he has advanced from Double-A Arkansas to Triple-A Salt Lake to The Show.
"I didn't see it happening this quick," he said. "I started the year in Double-A. Opportunities have presented themselves, and I've been able to take advantage.
"In this game, you have to be an extreme competitor. My philosophy is when we score, to put up a zero and keep the momentum on your side of the field."
Admittedly "pretty nervous" in San Francisco, O'Sullivan said he'll be relying on his teammates to help get him through again.
"We have a great defense behind us," O'Sullivan said. "I want to keep pitches down, in the zone, and let the defense make plays."