"Just got a little tendinitis in there," Rasmus said. "[I] talked to the doctors, and just with the intense nature of the beast that is baseball ... so I'm going to try and get it calmed down and look at figuring out how to make the pain go away."
Rasmus, who has not played since Sunday, was asked if he was optimistic the 10-day stint on the DL would be enough to get him back on the field.
"I am, for sure," Rasmus said. "... [I'll] try to get back as quick as I can. [I] just have tendinitis. It's something that occurs with the surgery. I had tendinitis in Spring Training, and it just kind of came back and jumped up on me when I stole a couple of bases.
"So it's something I played through, and then playing through it and trying to be tough ended up making it a little worse. I want to try and make sure I'm around for the rest of the season and not miss a couple of weeks or a month with something I could sit out a couple of days for to get going if I can."
Rays manager Kevin Cash echoed Rasmus' sentiments.
"We need Colby for the long run here," Cash said. "He's performed really well for us here, both offensively and defensively. [We] want to clean this thing up to where he's feeling good."
In Peterson's earlier stint with the Rays, he hit .263 with a home run and six RBIs.
"Shane Peterson played well for us," Cash said. "He played good defense. Got some big hits for us. So we're excited to have him back up here to fill in for that time. And he's going to get to play."
Worth noting The Rays got good news on Matt Duffy's progress. The shortstop had a procedure done in which a calcium deposit the size of a pea was removed from the area in his heel where he's been experiencing pain while trying to come back from heel surgery.
"We're hoping that is what's been causing all the irritation," Cash said, "because it was taken right from the spot where he's had most of his pain.
"I think [head athletic trainer] Ron Porterfield and the doctors think that once the stitches heal up, that he might see a quick turnaround. So we're being cautiously optimistic about that."
Catcher Wilson Ramos and right-hander Brad Boxberger each said they had completed their rehab assignments, so it's likely both will be activated from the DL this weekend.
Bill Chastain has covered the Rays for MLB.com since 2005.