San Francisco recalled rookie Chris Heston (11-7), who will pitch Sunday against St. Louis after getting a short break in the minors to rest. He last threw for the Giants on Aug. 17 at St. Louis and is winless in his past four starts.
Cain said he felt something in the elbow during his start Tuesday in a loss to the Cubs. He had an MRI exam Wednesday when the discomfort and "achy feeling" -- which he felt some while rehabilitating another elbow issue earlier this year -- lingered.
"My elbow looks good. Nothing out of the ordinary," he said. "It's just to try to calm that down and so I'm able to throw with a natural motion."
Heston threw a bullpen session Wednesday, so he's on schedule for leading up to his start. The Giants hope a short break helped Heston refresh ahead of what should be a pressure-packed September as the defending World Series champions chase another playoff berth.
"That's the whole goal," Heston said. "When I get into those October and November months, where we want to be playing still, and those innings start piling up, I think looking back it's going to be good that I took a little breather now."
The move with Cain was made retroactive to Wednesday. This marks his second stint on the DL this season after he began the year with an elbow injury and missed 79 games.
Cain is 2-4 with a 6.15 ERA in 10 starts this year.
"Just a little nerve irritation," manager Bruce Bochy said. "It's not worth the risk. He should be optimistic. I know our training staff, doctors are. We'll have him back throwing here soon."
Cain said he is willing to pitch in relief if that is best down the stretch in September, though Bochy didn't want to predict how he might be used.
Center fielder Gregor Blanco returned to the lineup after missing two games with a left hip strain. Angel Pagan, out with tendinitis in his right knee, will continue a rehab assignment with Class A San Jose on Saturday and be activated once rosters expand next week.
Shortstop Brandon Crawford missed his third straight game with tightness in his left side, which he said he was still experiencing Friday. Crawford and Bochy both indicated he would likely sit out the remainder of this weekend's series with the Cardinals and be ready for a key series at Dodger Stadium starting Monday.
"It's not something you want to rush," Crawford said. "Obliques are one of the most re-injured."
Second baseman Joe Panik, sidelined since Aug. 2 with low back inflammation, took ground balls and hit off a tee in the indoor cage. He is scheduled for batting practice in the cage Saturday, live BP on the field Sunday and could begin a rehab assignment with Triple-A Sacramento on Tuesday.
"Joe's making some progress," Bochy said.