The two moves set the 25-man roster after outfielder Jake Marisnick and right-hander Anthony DeSclafani were optioned to New Orleans.
Yelich went on the disabled list with a lower-back strain on June 16, retroactive to June 14.
"It was a very slow 15 days," the 22-year-old said. "It's good to be back out there and trying to help the team win."
Yelich basically dealt with back spasms, and he returned on the first day he was eligible to be reinstated.
Yelich started the series finale on Sunday against the A's in center field, because Marcell Ozuna was given an off-day after experiencing a right leg cramp.
In his first at-bat on Sunday, Yelich tripled and came around to score on an errant throw that got past Josh Donaldson as the speedy outfielder slid into third.
Ozuna says he is fine, and with Monday being a day off, he should be back in the lineup.
Yelich, back in the leadoff spot, is hitting .259 with a .342 on-base percentage in 63 games.
His lower back grabbed on him after fielding a routine single to left field on June 13 against the Pirates. He played one inning that game, before exiting.
"It's one of those things you've got to take it slow," Yelich said. "You probably take a couple of extra days to make sure there are no lingering effects. I'm definitely glad it's over with."
Yelich appeared in four rehab assignment games, two with Class A Jupiter and two with Triple-A New Orleans, before being reinstated.
When he returned to Miami on Saturday afternoon, he was a little bit empty handed. New Orleans was playing in Nashville, and Yelich checked a bag that included his glove and a Zephyrs cap. When he arrived in Miami, and opened his luggage, the glove and cap were gone.
As a replacement, Marisnick left Yelich his glove.
Marisnick's glove has his nickname, "Big Fudge" inscribed on the side.
Jennings adds a fresh arm to Miami's bullpen, which has logged 20 innings over the last three days.
The lefty is 0-1 with a 0.86 ERA with the Marlins this season.