Iwamura, who has been on the 15-day disabled list since late April because of a strained muscle in his side, has passed all of his tests on a rehab assignment at extended Spring Training.
Maddon added that Iwamura said he feels good to go for the opener of the Rays' three-game series against the Detroit Tigers at Tropicana Field.
"Part of this is trusting a veteran player," Maddon said. "He was adamantly positive that he felt good."
Maddon added that Iwamura will play third base, as usual, and will probably bat either sixth or seventh.
The 28-year-old rookie from Japan was hitting .339 with a home run, 15 runs scored and three stolen bases before going down on April 23.
Infielder Brendan Harris said the Rays have missed Iwamura in just about every way. Harris, who has been playing shortstop, will be glad to have Iwamura to his right on defense.
"He brings a lot to the table," Harris said. "He brings some speed, he's another left-handed bat, and a good one -- and great defense. He was on a roll so it was kind of tough [to lose him]."
The Rays did not say what kind of roster move would be made to make room for Iwamura, but sending down Jonny Gomes or Jorge Cantu is possible.
Rotation shuffle: Right-hander Jae Seo is out of the rotation for now, Maddon said. Seo has an 8.10 ERA and allows nearly two baserunners for every inning he pitches.
"Jae will be available in the bullpen," Maddon said. "He may [someday] go back to starting. We're not sure yet."
Right-hander Edwin Jackson, who had been pushed back two extra days coming into Saturday's rainout, will go Monday against the Tigers. Jackson, who is 0-6 with a 7.78 ERA, has been tinkering with his delivery with the help of pitching coach Jim Hickey.
Casey Fossum and James Shields will follow in the rotation -- the rest of which is to be decided, Maddon said.
The spy who caught me: Toby Hall, who spent parts of seven seasons with the Rays, started behind the plate for the White Sox on Sunday.
Maddon didn't think it gave the home team any kind of advantage against left-hander Scott Kazmir -- a former teammate of Hall. Maddon used an analogy featuring former UCLA basketball coach John Wooden, who said to "Take care of your own side first."
"If [Hall] knew something about Kaz tipping his pitches off, that would be one thing. That would be pertinent," Maddon said. "If Kaz has his good stuff going, he can tell the hitter what's coming and still be successful."
Hall recalled that Kazmir has struggled with his economy of pitches -- sometimes his pitch counts can reach the low 100s in only five or six innings -- but that the young lefty gets better with every start.
"The more experience he gets under his belt, he's going to be dominating," Hall said.
Riggans ready: Catcher Shawn Riggans received his first action and start of the season at catcher. Riggans had been active since Josh Paul went on the DL this past week, but Maddon had been cautious about using him because of a strained right elbow. Riggans batted .295 for Triple-A Durham.
Up next: The Rays head home on Monday to face the Detroit Tigers, the defending American League champions. Jackson, still looking for his first win of the season, opposes right-hander Chad Durbin at 7:10 p.m. ET.
David Brown is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.