CHICAGO -- Cubs manager Joe Maddon has emphasized that there is no established finish line for Craig Kimbrel's comeback from a right elbow issue. There is, however, a finite amount of time remaining in the regular season.

Kimbrel took another step forward on Sunday in his quest to rejoin Chicago's bullpen before the calendar flips to October. The closer completed a 20-pitch bullpen session, firing fastballs and curveballs without issue and registering what the Cubs felt were normal readings for a second mound session following a brief shutdown period.

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"It was good," Cubs pitching coach Tommy Hottovy said prior to Sunday's game against the Pirates. "In terms of him and intensity, as he's progressing, it's just gotten better over the last five or six days. So I think he's on par with what we were expecting for today, in terms of velocity and spin and all the stuff we expect in a normal bullpen. We'll just see how he recovers."

The next step for the 31-year-old Kimbrel is seeing how he feels when he reports to Wrigley Field on Monday, when the Cubs open a three-game series against the Reds. Chicago will monitor his progress in workouts and playing catch before lining up another mound session. Hottovy said that would likely be a simulated setting against hitters by the middle of next week.

"Hopefully everything progresses the way we want to in the next day or so," Hottovy said, "and we'll kind of reassess and try to get something going."

One silver lining has been the performance of Chicago's relief corps as a whole since Kimbrel was shelved. Dating back to Sept. 2 -- one day after Kimbrel's most recent appearance -- the Cubs' bullpen ranks second in the Majors in ERA (2.44), opponents' average (.173) and opponents' OPS (.571), while ranking third in strikeouts per nine innings (11.4).

While Kimbrel has been out, Maddon has mixed and matched at the end of games, using Rowan Wick and Kyle Ryan as his main high-leverage arms. Maddon has also turned to lefty Brad Wieck, in addition to Steve Cishek, Brandon Kintzler, Pedro Strop and Derek Holland.

Kimbrel was placed on the 10-day injured list on Sept. 5 (retroactive to Sept. 2) after experiencing right elbow inflammation, which was confirmed via an MRI exam. Initially, Kimbrel was optimistic about being able to return when eligible on Thursday. The Cubs originally projected this weekend as a possible target for activation.

Kimbrel's recovery has been slower than anticipated, but Cubs president of baseball operation Theo Epstein said earlier this weekend that the recent progress has the team hopeful about the closer's ability to return before the end of the season.

"Based on how he feels, [we're] pretty optimistic," Epstein said. "But we'll be cautious until we see the results of the bullpen."

Russell moved to concussion list

Prior to Sunday's game, the Cubs placed shortstop Addison Russell on Major League Baseball's seven-day concussion list (retroactive to Thursday). Russell's status is unchanged. He remains in MLB's concussion protocol and has not advanced to any baseball activities.