Manager Joe Maddon does not expect them to be down there for long.
"Stuff-wise and ability-wise, they can play here," Maddon said.
Tampa Bay also sent lefty Alex Torres down to the Minors.
Jennings, 23, earned widespread status as the team's best prospect after hitting .318 with 11 home runs and 52 stolen bases last season, splitting time between Double-A Montgomery and Triple-A Durham. He went 3-for-5 with a double and a run scored in three Grapefruit League games this spring, after clocking in at sixth overall on MLB.com's list of the 50 top prospects in baseball.
Jennings projects as the Rays' center fielder of the future.
"You could just watch him in batting practice and see how compact he is and how well everything works," Maddon said. "That was obvious."
Depending on the source, Hellickson, 22, ranks either just ahead or just behind Wade Davis on the club's top prospect list. A hard-throwing right-hander, Hellickson struck out 132 batters over 114 innings last year at Montgomery and Durham, while walking merely 29. He struck out six over 2 2/3 shutout innings this spring, walking none and allowing just one hit.
Hellickson ranked 20th on MLB.com's prospect list.
"It wasn't too big of a shock," Hellickson said. "I knew it was coming sometime, but no one wants that call into the office and told you're getting sent back down."
"He's going to be very good," Maddon said. "I like him a lot. Now that I've had a chance to see him and get to know him a little bit, he's got the right makeup to do it also."
In Maddon's estimation, Hellickson's fastball and changeup are Major League-ready, though his breaking ball "needs to be tightened up."
That process will occur at Triple-A, where Hellickson and Jennings are set to start the season. For Hellickson in particular, it will be a chance to gain some stamina by pitching every fifth day. And if Tampa Bay has holes to fill come midsummer, he and Jennings may be among the first players asked to fill them.
"I'm definitely itching to get up there," Hellickson said.
Torres, 22, is just a tick further away from the Majors, climbing only as high as Double-A in his fourth professional season. Ranked ninth on Baseball America's list of the club's top prospects, Torres features a four-seam and two-seam fastball, a curveball and a changeup, parlaying those pitches into a 2.86 ERA in his first tour of Double-A duty.
Torres was one of the main pieces that the Rays received from the Angels in exchange for Scott Kazmir last summer.
Anthony DiComo is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.