The Royals went 9-16 in Cain's absence and plummeted to fourth place in the American League Central.
"I'll do whatever it takes to get this team winning," Cain said before Friday's game. "Hopefully I can be a spark. That's a priority right now.
"It's been a tough season so far. We know we're capable of playing a lot better baseball."
To make room on the roster, the Royals optioned Brett Eibner to Omaha.
Manager Ned Yost said Cain likely would play in right field instead of center for a while to ease the amount of running Cain will have to do.
Cain said he is pretty close to 100 percent recovered.
"Everything felt great," Cain said. "I played seven innings the first night and nine innings the next night. It's time to get going.
"I wouldn't say my final gear is there, but it's close enough. We'll say it's 90 percent."
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.