Manager Ned Yost said it was a planned night off for Cain anyway and he was taking BP with the fourth group of extra players.
"When he was coming in, he said he felt a little something when he was playing catch, but didn't think anything about it," Yost said. "He came in and took a swing and felt it, took another swing and it was worse, and took the third swing and walked out. That's when it really started bothering him.
"We had no idea until I saw him walk off. I went upstairs, asked him how he was doing and he said, 'I'll be OK, I'll be all right.' Nick [Kenney, head trainer] came in, thought it was an oblique strain and he sent [Cain] once the game started to go get an MRI. We didn't get the results till late last night that showed it was definitely an oblique strain."
Last season, Cain was hit with three different leg injuries that limited him to 61 games. He had a left groin strain, a torn left hip flexor and a right hamstring strain. Cain spent the winter building up his legs, Yost gave him occasional days off as a precaution and he's had no leg problems this year.
The Royals set no timetable for Cain's return, but the recovery time of three to six weeks is typical.
"They are tricky," Yost said. "I don't have a feel on it. We've got 15 days to see where he's at. I just don't know."
The uncertainty makes the July 31 Trade Deadline acquisition of outfielder Justin Maxwell from the Houston Astros even more important for the Royals.
"Huge," Yost said.
With Cain out, Maxwell is the only right-handed-hitting outfielder on the roster along with left-handed-hitting Alex Gordon, Jarrod Dyson and David Lough.
"We've got the same glove in Dyson, and Lough does cover ground out there so you just deal with it," Yost said.
Gordon gives the Royals a Gold Glove Award winner in left field and Maxwell arrived with a good report as a defensive player.
In his first seven games for Kansas City, Maxwell was productive with a .471 (8-for-17) average, three home runs and six RBIs. He came from the Astros with a .266 average, five homers and 14 RBIs in 47 games.
Maxwell started in right field on Saturday night against Red Sox left-hander Felix Doubront and Dyson played center field.
Cain was hitting .261 in 96 games with 25 extra-base hits, 43 RBIs, 12 stolen bases and a .324 on-base percentage. His four home runs include his first grand slam. He's batted in every lineup spot except fourth and ninth, occasionally leading off recently.
His defensive play has been spectacular, highlighted by against-the-wall catches and strong throws resulting in seven assists.
The Royals already had five outfielders with infielder Elliot Johnson also available for outfield duty, so there was no immediate need to replace Cain with another outfielder.
"I still need another right-handed bat, but there's not one there right now so we'll just work through it," Yost said.
Cain's roster spot was taken by left-handed reliever Will Smith, who was recalled from Triple-A Omaha. At the same time, lefty reliever Francisley Bueno was designated for assignment and the club recalled lefty Donnie Joseph.
The DL stay for Cain is retroactive to Friday.