Capps has been out since May with a right elbow injury, and Jennings sustained a concussion after being struck in the head by a line drive at Pittsburgh on Aug. 7.
Jennings pitched Friday night in a rehab assignment with Class A Advanced Jupiter, and Capps is scheduled to throw on Saturday with the Hammerheads.
Because the Marlins aren't likely to make a deal by Sunday's waiver period trade deadline, the most significant additions will come from within.
Capps, who last pitched for Miami on May 25, has the chance to become a significant impact player down the stretch. Jennings is a lefty option.
"It gives us some more cushion, having more arms," manager Mike Redmond said. "It's always nice for every team in September to bring extra guys up. No doubt, it's the same for us. We'll have some guys who can pitch more innings and take some pressure off the guys we've had to rely so heavily on the whole year. That always strengthens your bullpen."
The Marlins' bullpen has been used heavily, logging 423 1/3 innings, which is the fifth most in the Majors.
In recent days, the Marlins received a scare when reliever Bryan Morris returned from Anaheim to Miami to have his right hip examined. It was determined that Morris has a groin strain, and he should be available on Monday as well.
The Marlins have four relievers who rank among the top 60 in the Majors in appearances. Mike Dunn paces the team with 62 appearances, followed by Steve Cishek (57), Morris (56) and A.J. Ramos (55).
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter