"Rich is a guy whose key attribute is his speed, and he uses [his speed] extremely well, both offensively and defensively," said Rays executive vice president of baseball operations Andrew Friedman. "We felt like profile-wise he fit in really well with us."
To make room on the active roster, outfielder Brandon Guyer was placed on the 15-day disabled list, retroactive to Sunday, with a left-shoulder strain. To accommodate Thompson's addition to the 40-man roster, right-hander Jeff Niemann was transferred from the 15-day disabled list to the 60-day DL.
"With where we're at right now with our second-string, third-string and fourth-string center fielders being on the DL, we felt it was really important to get someone we felt really comfortable with in center field," Friedman said.
Friedman said that in the event something happened to starting center field B.J. Upton, or if he needed a day off, Tampa Bay felt more comfortable looking outside the organization than switching Ben Zobrist or Elliot Johnson from their positions to play center field.
The Rays currently have nine players on the disabled list.
"We spent a lot of time in the offseason going through different scenarios and trying to insure that we have the requisite amount of depth in-house," Friedman said. "I think one of the things we overlooked was what if all of these things happened at one time, such a concentration of injuries in a short period of time. We needed to go outside the organization to fill this profile."
Thompson, 33, has spent 13 seasons in the Minor Leagues and has not appeared in the Major Leagues since 2004, when he played in six games for the Royals.
"It was pretty emotional," Thompson said of the trade. "I had been in Philadelphia for five years, so I had a lot of people in that organization that were really pulling for me. I got a call from the front office, from coaches that I've had there and everybody was just so positive. It meant a lot. A lot of support."
Thompson was hitting .307 with no home runs, 11 RBIs and seven stolen bases in 29 games for Triple-A Lehigh Valley, where he has spent the majority of the past five seasons.
The backup outfielder was thrust into action, as he served as a pinch-runner during the eighth inning of the Rays' 2-1 win over the Red Sox.
Were there any nerves?
"Surprisingly not," he said. "Everything kind of slows back when you get back on the field, but it's all the anticipation that makes you nervous. I might have been a little jumpy, kind of tipped my hand a little bit when I was trying to get my jump.
"I did draw a lot of attention to myself, so I guess it ended up working out OK."
Thompson has amassed 442 stolen bases in his Minor League career, including seven seasons of 40 or more steals, and an International League-leading 48 in 2011.
The left-handed-hitting Thompson has played all three outfield positions in the Minors, but he has played primarily center field.
Thompson made the Royals' Opening Day roster in 2004 and was used primarily as a pinch-runner and defensive replacement. He went 0-for-1 with one run scored and a stolen base. He was designated for assignment on April 24 that season and has not played in the Major Leagues since.
Guyer, 25, becomes the ninth Rays player on the disabled list. Since being recalled from Triple-A Durham on May 9, he appeared in three games, batting .143 with one home run and one RBI.
Guyer hurt himself while taking batting practice in Baltimore over the weekend. The Rays gave him a few days to see how well he felt before deciding to make a move.
"It became pretty clear he was going to need some time off," Friedman said, "so we started going through external players [to consider trading for]."
Niemann, 29, was placed on the 15-day disabled list Tuesday due to a fractured right fibula, an injury he sustained in Monday's game in Toronto. He is 2-3 with a 3.38 ERA in seven starts this season.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.