CHICAGO -- The Cubs' bullpen has been the best in baseball during the month of September to this point. And now the group has its anchor back, though his re-entry was a little rocky.

Prior to Thursday's key series opener against the Cardinals, the Cubs activated closer Craig Kimbrel from the 10-day injured list, following his bout with right elbow inflammation. The righty entered a 4-4 game to start the 10th inning and gave up the deciding homer to Matt Carpenter, whose shot handed Chicago a 5-4 loss.

"It's frustrating," Kimbrel said. "I've been doing everything I can to get back out there, and our team battled to the very end. We needed that. I'm pretty disappointed in myself to go out there and give up a home run like that. Tonight was a big game for us."

Kimbrel, who has been on the IL twice this season, hadn't pitched on Sept. 1 against the Brewers, giving up three runs in two-thirds of an inning and later informing the team about the discomfort in his elbow. An MRI exam confirmed inflammation, but no structural damage, and the right-hander built up to three mound workouts prior to activation.

Through 22 appearances this season, the 31-year-old righty has a 5.95 ERA with 13 saves, 28 strikeouts and 11 walks in 19 2/3 innings after signing a three-year, $43 million free-agent deal with the Cubs in June.

Entering Thursday, the Cubs' bullpen ranked first in the Majors in ERA (2.12), opponents' average (.182) and OPS (.553) as well as ground-ball rate (56.9 percent) dating back to Sept. 2 (after Kimbrel was shelved). Within that showing, rookie right-hander Rowan Wick has emerged as a trusted high-leverage weapon down the stretch.

Besides Wick, Cubs manager Joe Maddon's main setup options have been lefty Kyle Ryan and righty Steve Cishek, with a mix-and-match approach with the rest of his 15-man bullpen. That number includes righty Brandon Kintzler, who has been hindered much of this month with a mild left oblique strain. Kintzler's return could also be on the horizon.

"Spectacular. The guys have been great," Maddon said of the group's work with Kimbrel and Kintzler sidelined. "From what I understand, our bullpen this month has been as good as anybody's bullpen this month. So just look at the names and look what these guys have done. They've seized opportunity. This is going to make them all better down the road as they continue their professional careers."