Sweeney, a .302 career hitter, had not played since May 1. He missed 88 games with a bulging disc in his back. The five-time All-Star batted .314 with four homers, three doubles, 10 RBI and eight walks in 11 games during a rehabilitation stint with three Royals farm clubs.
"I had a great time playing in the minor leagues, but this is my team and I'm very excited to be back," Sweeney said. "We've had a rough year. I know I've had a rough year with my physical limitations of my back. I know we haven't played that well up to this point, but we still have two months to go. It's not how you start, but how you finish."
Sweeney hit .176 with two home runs and six RBI in 20 games before going on the DL.
"I've done everything I can to be healthy," Sweeney said. "I think I've waited ample time. I've done what the doctors have told me to do with the epidural shots and medications. Right now, it is time to go out and play ball and have fun. I'm going to play as hard as I can every day and hopefully my back will hold up and the last two months will be fun. Hopefully these last two months I can make an impact in Kansas City."
The Royals have sorely missed Sweeney's bat. They ranked 13th in the American League with a .401 slugging percentage and 12th with a .265 batting average.
"Mike is back," manager Buddy Bell said. "It is nice. I haven't been able to write him in the lineup for a while."
Bell said he plans to give Sweeney a day or two off now and then to rest him.
In other roster moves, the Royals optioned infielder Andres Blanco and right-hander Leo Nunez to Triple-A Omaha. Blanco hit .219 in 10 games, while Nunez was 0-0 with a 4.73 ERA in seven relief appearances. The Royals recalled left-hander Andrew Sisco from Omaha, where he had allowed one run in 4 2/3 innings after being optioned on July 27.
The Royals also transferred right-hander Scott Elarton, who had season-ending shoulder surgery last week, to the 60-day disabled list.