Balfour sustained the injury before Friday's game while horsing around with pitching coach Jim Hickey. The two were on the field before batting practice, "just going about messing around," Maddon said.
"It's unfortunate. He had been pitching so well. He's a big part of what we're doing," Maddon added. "He's looked a lot like he did a couple years ago. We're going to miss him, definitely going to miss him. I'd like to have him and ... Chad Qualls here, too."
Trading for Qualls, a right-handed reliever, on Saturday helps solidify the bullpen until Balfour returns, but the trio of Balfour, setup man Joaquin Benoit and closer Rafael Soriano had developed into a devastating late-game combination. Balfour had made an impressive return to his 2008 form this year with a 2.08 ERA and 44 strikeouts to just 13 walks in 43 1/3 innings of work.
"Obviously, Balfour is a big part of our success, and I think it's an area that we have some depth, but you'd rather it be depth," said Andrew Friedman, executive vice president of baseball operations. "I think it's definitely not a good thing. Hopefully [we can] get him back, he'll be fresh and will be able to impact us over the last two to four weeks of the season and hopefully into October."
But the added depth and Balfour's potential return don't take the sting out of the injury, especially given the timing and the manner in which it happened. Friedman said no disciplinary actions will be taken against Balfour or Hickey because both were just kidding around and doing nothing different than what happens in all Major League clubhouses.
"That being said, it was obviously very frustrating and obviously very avoidable now that you look back on it in retrospect," Friedman said. "Guys mess around like they do thousands of times, and nothing ever happens, but when it does, hopefully it makes people more conscious of that, because we have a chance to do special things this year. That's the part of it that's frustrating."
With Balfour heading to the DL, the Rays will not have to make a roster move to account for the addition of Qualls just yet. Given the timing of Balfour's rehabilitation, he may not return until after Sept. 1, when Major League rosters expand to 40 players.
Adam Berry is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.