"It's always disappointing to go to the DL, no matter what happens, no matter how close you get [to recovery]. It's just frustrating," said Jennings, who hopes that added rest will enable him to return to action in eight days, when he's eligible.
Butler, 29, was slated at designated hitter for the Rays and batted fifth in Sunday's series finale against Baltimore.
"It feels good. I'm excited," said Butler moments after arriving at Tropicana Field. "I feel like I've gained a little more experience, so I'm excited to see how things play out."
Butler has appeared in 14 Major League games over parts of two seasons with Texas (2013) and St. Louis (2014), going 4-for-17 with two doubles and an RBI. Butler was leading Durham in hits (26), RBIs (15) and total bases (40) at the time of his promotion.
Jennings was a last-minute scratch from Saturday night's game after originally being slated to serve as the DH.
"Before the game, it was hurting. It's been hurting, but I thought I could go out and DH and play, but I couldn't ," said Jennings.
The 28-year-old outfielder has not played since being forced to leave last Saturday's game vs. the Blue Jays with knee soreness.
"I don't want to say it wasn't getting better. It just wasn't getting better fast," said Jennings.
On the season, Jennings is hitting .222 with no home runs and two RBIs in 18 games.